Light-emitting device

ABSTRACT

There is provided an EL light-emitting device with less uneven brightness. When a drain current of a plurality of current controlling TFTs is Id, a mobility is μ, a gate capacitance per unit area is Co, a maximum gate voltage is Vgs (max) , a channel width is W, a channel length is L, an average value of a threshold voltage is Vth, a deviation from the average value of the threshold voltage is ΔVth, and a difference in emission brightness of a plurality of EL elements is within a range of ±n %, a semiconductor display device is characterized in that 
     
       
         
           
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CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/010,118, filed Jan. 20, 2011, now allowed, which is a continuation ofU.S. application Ser. No. 11/553,197, filed Oct. 26, 2006, now U.S. Pat.No. 7,995,010, which is a continuation of U.S. application Ser. No.10/600,866, filed Jun. 23, 2003, now U.S. Pat. No. 7,129,917, which is adivisional of U.S. application Ser. No. 09/796,412, filed Feb. 27, 2001,now U.S. Pat. No. 6,583,776, which claims the benefit of a foreignpriority application filed in Japan as Serial No. 2000-054963 on Feb.29, 2000, all of which are incorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an EL panel in which an EL elementformed on a substrate is sealed between the substrate and a covermember. Further, the present invention relates to an EL module in whichan IC is mounted in the EL panel. Incidentally, in the presentspecification, the EL panel and the EL module are generally referred toas a light-emitting device. The present invention further relates to anelectronic instrument using the light-emitting device.

2. Description of the Related Art

In recent years, a technique for forming a TFT on a substrate has beengreatly advanced, and application to an active matrix display device hasbeen advanced. Especially, since a TFT using a polysilicon film has anelectron field-effect mobility (also called mobility) higher than thatof a TFT using a conventional amorphous silicon film, a high speedoperation is possible. Thus, control of a pixel, which is conventionallyperformed by a driving circuit outside a substrate, can be performed bya driving circuit formed on the same substrate as the pixel.

In this sort of active matrix display device, various merits, such asreduction of manufacturing costs, miniaturization of an electro-opticdevice, improvement of a yield, and reduction of a throughput, can beobtained by forming various circuits and elements on the same substrate.

Further, research of an active matrix type light-emitting deviceincluding an EL element as a self-luminous element has been activelycarried out. The light-emitting device (EL display) including the ELelement is also called an organic EL display (OELD: Organic EL Display)or an organic light-emitting diode (OLED: Organic Light-emitting Diode).

The light-emitting device is of a self-luminous type differently from aliquid crystal display device. The EL element has such a structure thata layer (hereinafter referred to as an EL layer) containing an organiccompound is sandwiched between a pair of electrodes (anode and cathode),and the EL layer has normally a laminate structure. Typically, there iscited a laminate structure “hole transporting layer/light-emittinglayer/electron transporting layer” proposed by Tang et al. of EastmanKodak Company. This structure has a very high luminous efficiency, andmost of the light-emitting devices on which research and development hasbeen made at present adopt this structure.

In the EL element, luminescence (Electro Luminescence) generated byapplication of an electric field is obtained, and it includes an anodelayer, an EL layer, and a cathode layer. Luminescence in an organiccompound includes light emission (fluorescence) generated when a singleexcited state returns to a ground state and light emission(phosphorescence) generated when a triplet excited state returns to theground state, and the EL display of the present invention may use eitherlight emission.

In addition, there may be also adopted a structure in which laminatingis made on an anode in the order of a hole injecting layer/a holetransporting layer/a light-emitting layer/an electron transporting layeror a hole injecting layer/a hole transporting layer/a light-emittinglayer/an electron transporting layer/an electron injecting layer. Thelight-emitting layer may be doped with a fluorescent pigment or thelike.

In the present specification, all layers provided between a cathode andan anode are generally referred to as an EL layer. Thus, all of theforegoing hole injecting layer, hole transporting layer, light-emittinglayer, electron transporting layer, electron injecting layer, and thelike are included in the EL layer.

Besides, in the present specification, an element formed of an anode, anEL layer and a cathode is referred to as an EL element.

In a light-emitting device, a plurality of pixels are provided in amatrix form, and each of the plurality of pixels includes a thin filmtransistor (TFT) and an EL element. FIG. 4 is a circuit diagram of apixel of a general light-emitting device. A pixel 400 includes aswitching TFT 401, a current controlling TFT 402, an EL element 403, asource signal line 404, a gate signal line 405, a power supply line 406,and a capacitor 407.

A gate electrode of the switching TFT 401 is connected to the gatesignal line 405. One of a source region and a drain region of theswitching TFT 401 is connected to the source signal line, and the otheris connected to a gate electrode of the current controlling TFT 402. Asource region of the current controlling TFT 402 is connected to thepower supply line 406, and a drain region is connected to an anode or acathode of the EL element 403.

In the case where the anode of the EL element 403 is connected to thedrain region of the current controlling TFT 402, the anode of the ELelement 403 becomes a pixel electrode, and the cathode becomes a counterelectrode. On the contrary, in the case where the cathode of the ELelement 403 is connected to the drain region of the current controllingTFT 402, the anode of the EL element 403 becomes the counter electrode,and the cathode becomes the pixel electrode.

Note that, in the present specification, a potential difference betweena potential of a pixel electrode and a potential of a counter electrodeis called an EL driving voltage, and this EL driving voltage is appliedto the EL layer.

Note that, as shown in FIG. 4, the capacitor 407 is provided to beconnected to the current controlling TFT 402 and the power supply line406.

The potential (power source potential) of the power supply line 406 iskept constant. The potential of the counter electrode of the EL element403 is also kept constant. The potential of the counter electrode has apotential difference from the power source potential to such a degreethat the EL element emits light when the power source potential isapplied to the pixel electrode of the EL element.

The switching TFT 401 comes to have an on state by a selection signalinputted to the gate signal line 405. Incidentally, in the presentspecification, that a TFT comes to have an on state means that a draincurrent of the TFT comes to have a state of more than 0.

When the switching TFT 401 comes to have the on state, a video signalinputted from the source signal line 404 is inputted to the gateelectrode of the current controlling TFT 402 through the switching TFT401. Incidentally, in the present specification, the video signal meansan analog signal including image information. Incidentally, that asignal is inputted to the gate electrode of the current controlling TFT402 through the switching TFT 401 means that a carrier moves through anactive layer of the switching TFT 401, and a potential of a video signalis given to the gate electrode of the current controlling TFT 402.

The amount of current flowing through the channel formation region ofthe current controlling TFT 402 is controlled by a gate voltage Vgs of apotential difference between the gate electrode and the source region ofthe current controlling TFT 402. Thus, the potential given to the pixelelectrode of the EL element 403 is determined by the height of thepotential of the video signal inputted to the gate electrode of thecurrent controlling TFT 402. The emission luminance of the EL element(luminance of light emitted from the EL element) is controlled by theheight of the potential given to the pixel electrode. That is, theluminance of the EL element 403 is controlled by the potential of thevideo signal inputted to the source signal line 404 and a gradationdisplay is carried out.

FIG. 5 shows the relation between the emission luminance (cd/m²) of anEL element and the current density (mA/cm²). The relation between theemission luminance of the EL element and the current density is linear.That is, when the current density of the EL element becomes high at aconstant rate, the emission luminance of the EL element also becomeshigh at a constant rate. The current density is determined by a draincurrent Id of the current controlling TFT 402.

Although it is desirable that TFTs formed in a pixel portion of alight-emitting device have the same characteristics, actually, thecharacteristics of the respective TFTs are subtly different from oneanother. Particularly, threshold Vth of a TFT is influenced by adifference in crystallinity of an active layer, an impurityunintentionally mixed in the active layer, and the like. Thus, there hasbeen a case where Vth is different among the TFTs. Incidentally, in thepresent specification, the active layer means a semiconductor filmincluding a source region, a drain region, and a channel forming regionof a TFT.

When the value of the threshold Vth of the TFT becomes different, thevalue of the drain current Id also becomes different. Expression 1indicates the relation between the drain current Id and the thresholdVth.

[Expression 1]

Where, μ (m²/V·sec) indicates a mobility of the TFT, and Co(F/cm²)indicates a capacitance value per unit area of a capacitance (gatecapacitance) formed by a gate electrode, an active layer and a gateinsulating film of the TFT.

Besides, W and L indicate a channel width and a channel length of achannel forming region of the TFT, respectively, and its position isshown in FIG. 6. FIG. 6 is a view schematically showing the TFT, and theactive layer includes a channel forming region 601, a source region 602,and a drain region 603. The channel forming region 601 is provided to besandwiched between the source region 602 and the drain region 603.Although not shown in FIG. 6, there is also a case where an LDD regionis provided between the channel forming region 601 and the source region602 or the drain region 603.

A gate electrode 604 is provided over the channel forming region 601through a gate insulating film (not shown). Note that in the presentspecification, the channel forming region 601 is included in a portionof an active layer 600 overlapping with the gate electrode 604 andindicates a portion where a channel is actually formed when a voltage isapplied to the gate electrode 604.

The channel length L is a length of the channel forming region in thedirection in which a carrier of a free electron or free hole flows. Thechannel width W is a length of the channel forming region in thedirection vertical to the direction in which the carrier flows. Althoughthe TFT shown in FIG. 6 has a single gate structure, in the case of aTFT having a multigate structure such as a double gate structure or atriple gate structure, the channel length L is defined as the sum oflengths of channel forming regions formed under all gate electrodes inthe direction in which the carrier flows.

As indicated by the expression 1, when the value of the thresholdvoltage Vth is varied, the value of the drain current Id is also varied.Thus, if the value of the threshold voltage Vth of the currentcontrolling TFT is different among pixels, even if video signals havingthe same potential are inputted to the respective pixels, the emissionluminance of the EL element becomes different among the pixels. Notethat in the present specification, to input a signal to a pixel means toinput a signal to a gate electrode of a current controlling TFT througha switching TFT included in the pixel.

If the emission luminance is not uniform in all pixels of thelight-emitting device, unevenness of luminance (uneven luminance)appears in an image displayed on the pixel portion and is visuallyrecognized by an observer.

In order to suppress the foregoing uneven luminance, as shown in FIG.18, a light-emitting device having a structure in which four TFTs areprovided in a pixel is devised (SID'98 DIGEST 4.2 “Design of an ImprovedPixel for a Polysilicon Active-Matrix Organic LED Display” R. M. A.Dawson etc.).

In FIG. 18, reference numeral 1701 designates a first thin filmtransistor; 1702, a second thin film transistor; 1703, a third thin filmtransistor; and 1704, a fourth thin film transistor. The emissionluminance of an EL element 1705 is controlled by the first to fourthfour thin film transistors.

When the first thin film transistor 1701 comes to have the on state by aselection signal inputted to a gate signal line (G), and the third thinfilm transistor 1703 comes to have the on state by a signal inputted toa first signal line (AZ), a gate electrode and a drain region of thesecond thin film transistor 1702 are short-circuited. Since the fourththin film transistor 1704 is in an off state by a signal inputted to asecond signal line (AZB), a gate voltage Vgs of a voltage between thegate electrode and a source region of the second thin film transistor1702 enters into a subthreshold region determined by a leak current.

Next, the third thin film transistor 1703 comes to have the off state bya signal inputted to the first signal line (AZ). Then, a video signal isinputted to a source signal line (S) and a potential of the video signalis given to the gate electrode of the second thin film transistor 1702through the first thin film transistor 1701 having the on state.Accordingly, the gate voltage Vgs of the third thin film transistor 1703becomes a potential obtained by adding the potential of the video signalto the gate voltage Vgs having entered into the subthreshold region.

Next, the first thin film transistor 1701 comes to have the off state bya selection signal inputted to the gate signal line (G). Then, thefourth thin film transistor 1704 comes to have the on state by a signalinputted to the second signal line (AZB). Since a current flowingthrough the channel forming region of the TFT depends on the value ofthe gate voltage Vgs of the third thin film transistor 1703, the currenthaving the intensity corresponding to the potential of the video signalis inputted to a pixel electrode of the EL element 1705.

In the case of the light-emitting device having the above structure, inthe case where video signals having the same potential are inputted tothe source signal line, it is possible to prevent the potential given tothe pixel electrode from being varied by the value of the threshold Vthof the second thin film transistor 1702. Thus, the uneven luminance ofan image can be suppressed. However, if the number of thin filmtransistors provided in each pixel is increased, the opening ratio islowered, and it becomes necessary to increase a current flowing throughan EL element in order to obtain constant luminance. If the currentflowing through the EL element is increased, deterioration of the ELlayer is accelerated, which is not preferable.

Besides, if the number of TFTs provided in a pixel is increased, thereis a tear that yield of the light-emitting device itself is lowered.

SUMMARY OF THE INVENTION

In view of the above, the present invention has an object to provide alight-emitting device in which the number of thin film transistorsprovided in each of pixels is restricted to two, and uneven luminancedue to fluctuation in threshold voltage of current controlling TFTsincluded in the respective pixels can be suppressed.

The present inventors have considered that it is necessary to restrict adifference in emission luminance of respective pixels provided in apixel portion to a certain constant range (for example, within ±5%) inorder to prevent uneven luminance of an image from being visuallyrecognized by an observer. Further, since uneven luminance is morenoticeable between adjacent pixels, the present inventor et al. haveconsidered that it is necessary that the difference in emissionluminance between adjacent pixels is restricted to a range (for example,within ±3%) narrower than the difference of emission luminance betweenpixels which are not adjacent to each other.

For example, in order to restrict the difference in the emissionluminance of the respective pixels to a range of ±n %, the followingexpression can be derived from the expression 1. When the expression 1is modified, expression 2 is obtained.

[Expression 2]

A mobility μ and a capacitance value Co of a gate capacitance are valuesfixed at the point of time when a TFT is formed. When an EL element ismade to emit light at desired emission luminance, since the relationbetween the emission luminance of the EL element and current density islinear, the value of a drain current Id is also fixed. Thus, expression3 is derived by replacing the right side of the expression 2 by aconstant A.

[Expression 3]

In consideration of confining the difference in the emission luminanceof the respective pixels to the range of ±n %, expression 4 andexpression 5 are obtained from the expression 3. Threshold voltage Vthis an average of threshold voltages of current controlling TFTs of allpixels. The symbol ΔVth stands for a difference between an actualthreshold voltage of each pixel and the threshold voltage Vth.

[Expression 4]

[Expression 5]

If Vgs−Vth=V′, expression 6 is derived from the expression 4 and theexpression 5.

[Expression 6]

Here, expression 7 is obtained from the expression 3.

[Expression 7]

Thus, expression 8 is derived from the expression 6 and the expression7.

[Expression 8]

When the expression 8 is solved with respect to W/L, expression 9 isobtained.

[Expression 9]

If the gate voltage Vgs is too high, the TFT itself is deteriorated, sothat it is necessary that the gate voltage Vgs has such an intensitythat an element is not broken. When a value of the gate voltage Vgsimmediately before the element is broken is made Vgs_((max)), thefollowing expression 10 is derived from the expression 3. Note that itis necessary that Vgs_((max)) is about 25 V, and is desirably 10 V orless.

[Expression 10]

Expression 11 is obtained from the above expressions 9 and 10.

[Expression 11]

If the values of ΔVth and W/L are determined in the range where theabove expression 8 or 11 is satisfied, the fluctuation of the draincurrent Id can be suppressed to the range of ±n %.

For example, in the case where the value of the ratio W/L of the channelwidth W to the channel length L is fixed by a problem of design, therange of the fluctuation ΔVth of the threshold voltage is determined bythe expression 8 from the value of the ratio W/L of the channel width Wto the channel length L.

In the case where the fluctuation ΔVth of the threshold voltage is fixedby a fabricating process of TFTs, the range of the ratio W/L of thechannel width W to the channel length L is determined by the expression11 from the value of the fluctuation ΔVth of the threshold voltage.

By the above structure, in the light-emitting device of the presentinvention, the number of thin film transistors provided in each ofpixels is made two to prevent a drop in an opening ratio, and it becomespossible to suppress uneven luminance due to fluctuation in thethreshold voltage of the current controlling TFT included in each of thepixels.

Note that the above expressions 4 to 11 are obtained under theassumption that the difference in the emission luminance of therespective pixels is restricted to the range of ±n %. In the case wherethe difference in the emission luminance between adjacent pixels isrestricted to the range of ±5%, the relation between the fluctuationΔVth of the threshold voltage and the ratio W/L of the channel width Wto the channel length L is expressed by the following expressions 12 and13.

[Expression 12]

[Expression 13]

If the values of ΔVth and W/L are determined within the range where theabove expression 12 or 13 is satisfied, the fluctuation of the draincurrent Id can be suppressed to the range of ±5%.

For example, in the case where the fluctuation ΔVth of the thresholdvoltage is fixed by a fabricating process of TFTs, the range of theratio W/L of the channel width W to the channel length L is determinedby the expression 12 from the value of the fluctuation ΔVth of thethreshold voltage.

Besides, in the case where the value of the ratio W/L of the channelwidth W to the channel length L is fixed by a problem of design, therange of the fluctuation ΔVth of the threshold voltage is determined bythe expression 13 from the value of the ratio W/L of the channel width Wand the channel length L.

By the above structure, in the light-emitting device of the presentinvention, the number of thin film transistors provided in each of thepixels is made two to prevent a drop in the opening ratio, and itbecomes possible to suppress uneven luminance due to fluctuation inthreshold voltage of current controlling TFTs included in the respectivepixels.

In the case where the difference in the emission luminance of therespective pixels is restricted to the range of ±3%, the relationbetween the fluctuation ΔVth of the threshold voltage and the ratio W/Lof the channel width W to the channel length L is expressed by thefollowing expressions 14 and 15.

[Expression 14]

[Expression 15]

If the values of ΔVth and W/L are determined within the range where theabove expression 14 or 15 is satisfied, the fluctuation of the draincurrent Id can be suppressed to the range of ±3%.

For example, in the case where the fluctuation ΔVth of the thresholdvoltage is fixed by a fabricating process of TFTs, the range of theratio W/L of the channel width W to the channel length L is determinedby the expression 14 from the value of the fluctuation ΔVth of thethreshold voltage.

Besides, in the case where the value of the ratio W/L of the channelwidth W to the channel length L is fixed by a problem of design, therange of the fluctuation ΔVth of the threshold voltage is determined bythe expression 15 from the value of the ratio W/L of the channel width Wto the channel length L.

By the above structure, in the light-emitting device of the presentinvention, the number of thin film transistors provided in each ofpixels is made two to suppress a drop in the opening ratio, and itbecomes possible to suppress uneven luminance due to fluctuation inthreshold voltage of current controlling TFTs included in the respectivepixels.

The structure of the present invention is as follows:

According to the present invention, there is provided a light-emittingdevice including a plurality of pixels, wherein:

the plurality of pixels include a plurality of switching TFTs, aplurality of current controlling TFTs, and a plurality of EL elements,

emission luminance of the EL elements are controlled by video signalsinputted to gate electrodes of the plurality of current controlling TFTsthrough the plurality of switching TFTs,

the plurality of current controlling TFTs respectively include activelayers, gate insulating films on the active layers, and gate electrodeson the gate insulating films,

the active layers respectively include source regions, drain regions,and channel forming regions provided between the source regions and thedrain regions, and

when a drain current of the plurality of current controlling TFTs whenthe luminance of the EL element becomes maximum is Id, a mobility is μ,a gate capacitance per unit area is Co, a maximum gate voltage isVgs_((max)), a channel width is W, a channel length is L, an averagevalue of a threshold voltage is Vth, a deviation from the average valueof the threshold voltage is ΔVth, and a difference in the emissionluminance of the plurality of EL elements is within a range of ±n %,Expression 16 is satisfied.

[Expression 16]

According to the present invention, there is provided a light-emittingdevice including a plurality of pixels, wherein:

the plurality of pixels include a plurality of switching TFTs, aplurality of current controlling TFTs, and a plurality of EL elements,

emission luminance of the EL elements are controlled by video signalsinputted to gate electrodes of the plurality of current controlling TFTsthrough the plurality of switching TFTs,

the plurality of current controlling TFTs respectively include activelayers, gate insulating films on the active layers, and gate electrodeson the gate insulating films,

the active layers respectively include source regions, drain regions,and channel forming regions provided between the source regions and thedrain regions, and

when a drain current of the plurality of current controlling TFTs whenthe luminance of the EL element becomes maximum is Id, a mobility is μ,a gate capacitance per unit area is Co, a maximum gate voltage isVgs_((max)), a channel width is W, a channel length is L, an averagevalue of a threshold voltage is Vth, a deviation from the average valueof the threshold voltage is ΔVth, and a difference in the emissionluminance of the plurality of EL elements is within a range of ±n %,Expression 17 is satisfied.

[Expression 17]

According to the present invention, a light-emitting device includes asource signal line driving circuit, a gate signal line driving circuit,a pixel portion, a plurality of source signal lines, a plurality of gatesignal lines, and power supply lines, wherein:

the pixel portion includes a plurality of pixels,

the plurality of pixels respectively include a plurality of switchingTFTs, a plurality of current controlling TFTs, and a plurality of ELelements,

the EL elements respectively include anodes, cathodes, and EL layersprovided between the cathodes and the anodes,

gate electrodes of the plurality of switching TFTs are connected to theplurality of gate lines,

ones of source regions and drain regions of the plurality of switchingTFTs are connected to the plurality of source signal lines, and theother ones are connected to gate electrodes of the plurality of currentcontrolling TFTs,

source regions of the plurality of current controlling TFTs areconnected to the power supply lines, and drain regions are connected tothe anodes or the cathodes of the EL elements,

video signals are inputted to the plurality of source signal lines bythe source signal line driving circuit,

the video signals inputted to the plurality of source signal lines areinputted to the gate electrodes of the plurality of current controllingTFTs through the plurality of switching TFTs so that emission luminanceof the plurality of EL elements is controlled,

the plurality of current controlling TFTs respectively include activelayers, gate insulating films on the active layers, and gate electrodeson the gate insulating films,

the active layers respectively include source regions, drain regions,and channel forming regions provided between the source regions and thedrain regions, and

when a drain current of the plurality of current controlling TFTs whenthe luminance of the EL element becomes maximum is Id, a mobility is μ,a gate capacitance per unit area is Co, a maximum gate voltage isVgs_((max)), a channel width is W, a channel length is L, an averagevalue of a threshold voltage is Vth, a deviation from the average valueof the threshold voltage is ΔVth, and a difference in the emissionluminance of the plurality of EL elements is within a range of ±n %,Expression 18 is satisfied.

[Expression 18]

According to the present invention, a light-emitting device includes asource signal line driving circuit, a gate signal line driving circuit,a pixel portion, a plurality of source signal lines, a plurality of gatesignal lines, and power supply lines, wherein

the pixel portion includes a plurality of pixels,

the plurality of pixels respectively includes a plurality of switchingTFTs, a plurality of current controlling TFTs, and a plurality of ELelements,

the EL elements respectively include anodes, cathodes, and EL layersprovided between the cathodes and the anodes,

gate electrodes of the plurality of switching TFTs are connected to theplurality of gate lines,

ones of source regions and drain regions of the plurality of switchingTFTs are connected to the plurality of source signal lines, and theother ones are connected to gate electrodes of the plurality of currentcontrolling TFTs,

source regions of the plurality of current controlling TFTs areconnected to the power supply lines, and drain regions are connected tothe anodes or the cathodes of the EL elements,

video signals are inputted to the plurality of source signal lines bythe source signal line driving circuit,

the video signals inputted to the plurality of source signal lines areinputted to the gate electrodes of the plurality of current controllingTFTs through the plurality of switching TFTs so that emission luminanceof the plurality of EL elements is controlled,

the plurality of current controlling 11. Is respectively include activelayers, gate insulating films on the active layers, and gate electrodeson the gate insulating films,

the active layers respectively include source regions, drain regions,and channel forming regions provided between the source regions and thedrain regions, and

when a drain current of the plurality of current controlling TFTs whenthe luminance of the EL element becomes maximum is Id, a mobility is μ,a gate capacitance per unit area is Co, a maximum gate voltage isVgs_((max)), a channel width is W, a channel length is L, an averagevalue of a threshold voltage is Vth, a deviation from the average valueof the threshold voltage is ΔVth, and a difference in the emissionluminance of the plurality of EL elements is within a range of ±n %,Expression 19 is satisfied.

[Expression 19]

The light-emitting device may be characterized in that the currentcontrolling TFTs are n-channel TFTs and the drain regions of the currentcontrolling TFTs are connected to the cathodes of the EL elements.

The light-emitting device may be characterized in that the currentcontrolling TFTs are p-channel TFTs and the drain regions of the currentcontrolling TFTs are connected to the anodes of the EL elements.

According to the present invention, there is provided a light-emittingdevice including a plurality of pixels, wherein:

the plurality of pixels include a plurality of switching TFTs, aplurality of current controlling TFTs, and a plurality of EL elements,

emission luminance of the EL elements is controlled by video signalsinputted to gate electrodes of the plurality of current controlling TFTsthrough the plurality of switching TFTs,

the plurality of current controlling TFTs respectively include activelayers, gate insulating films on the active layers, and gate electrodeson the gate insulating films,

the active layers respectively include source regions, drain regions,and channel forming regions provided between the source regions and thedrain regions,

when a drain current of the plurality of current controlling TFTs whenthe luminance of the EL element becomes maximum is Id, a mobility is μ,a gate capacitance per unit area is Co, a maximum gate voltage isVgs_((max)), a channel width is W, a channel length is L, an averagevalue of a threshold voltage is Vth, a deviation from the average valueof the threshold voltage is ΔVth, and a difference in the emissionluminance of the plurality of EL elements is within a range of ±n %,Expression 20 is satisfied, and

a ratio of the channel width W to the channel length L in each of thepixels is different from one another according to a color displayed byeach of the pixels.

[Expression 20]

According to the present invention, there is provided a light-emittingdevice including a plurality of pixels, wherein:

the plurality of pixels include a plurality of switching TFTs, aplurality of current controlling TFTs, and a plurality of EL elements,

emission luminance of the EL elements is controlled by video signalsinputted to gate electrodes of the plurality of current controlling TFTsthrough the plurality of switching TFTs,

the plurality of current controlling TFTs respectively include activelayers, gate insulating films on the active layers, and gate electrodeson the gate insulating films,

the active layers respectively include source regions, drain regions,and channel forming regions provided between the source regions and thedrain regions,

when a drain current of the plurality of current controlling TFTs whenthe luminance of the EL element becomes maximum is Id, a mobility is μ,a gate capacitance per unit area is Co, a maximum gate voltage isVgs_((max)), a channel width is W, a channel length is L, an averagevalue of a threshold voltage is Vth, a deviation from the average valueof the threshold voltage is ΔVth, and a difference in the emissionluminance of the plurality of EL elements is within a range of ±n %,Expression 21 is satisfied, and

a ratio of the channel width W to the channel length L in each of thepixels is different from one another according to a color displayed byeach of the pixels.

[Expression 21]

The light-emitting device may be characterized in that the difference inthe emission luminance of the plurality of EL elements is within a rangeof ±5%.

The light-emitting device may be characterized in that the difference inthe emission luminance of the plurality of EL elements is within a rangeof ±3%.

The light-emitting device may be characterized in that the maximum gatevoltage is 25 V.

The light-emitting device may be characterized in that the maximum gatevoltage is 25 V and a ratio of the channel width W to the channel lengthL of each of the plurality of current controlling TFTs is2.26×10⁻³≦W/L≦0.214.

The gate capacitance is formed in a portion where the channel formingregion, the gate insulating film, and the gate electrode overlap withone another in each of the current controlling TFTs.

A video camera characterized by using the light-emitting device.

An image reproduction apparatus characterized by using thelight-emitting device.

A head mount display characterized by using the light-emitting device.

A personal computer characterized by using the light-emitting device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a pixel portion of a light-emittingdevice of Embodiment 1;

FIG. 2 is an upper plane block diagram of the light-emitting device ofEmbodiment 1;

FIG. 3 is a timing chart showing a driving method of the light-emittingdevice of Embodiment 1;

FIG. 4 is a circuit diagram of a light-emitting device of the presentinvention;

FIG. 5 is a correlation view of emission luminance of an EL element andcurrent density;

FIG. 6 is a top view of a TFT;

FIGS. 7A to 7D are views showing fabricating steps of a light-emittingdevice of Embodiment 3;

FIGS. 8A to 8D are views showing fabricating steps of the light-emittingdevice of Embodiment 3;

FIGS. 9A to 9C are views showing fabricating steps of the light-emittingdevice of Embodiment 3;

FIGS. 10A and 10B are views showing fabricating steps of thelight-emitting device of Embodiment 3;

FIGS. 11A and 11B are a top view and a sectional view of alight-emitting device of Embodiment 4;

FIGS. 12A to 12C are circuit diagrams of pixels of light-emittingdevices of Embodiment 5;

FIG. 13 is a circuit diagram of a source signal line driving circuit ofEmbodiment 6;

FIGS. 14A and 14B are equivalent circuit diagrams of a level shift andan analog switch of Embodiment 6;

FIG. 15 is a top view of a pixel of Embodiment 7;

FIGS. 16A to 16F are views of electronic apparatuses each using alight-emitting device of Embodiment 13;

FIGS. 17A and 17B are views of electronic apparatuses each using alight-emitting device of Embodiment 13;

FIG. 18 is a circuit diagram of a pixel portion of a conventionlight-emitting device;

FIG. 19 is a view of a spin coater used when a light-emitting device ofEmbodiment 8 is fabricated; and

FIG. 20 is a sectional detailed view of a light-emitting device ofEmbodiment 10.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Embodiments of the present invention will be described below.

Embodiment 1

In this embodiment, an example in which the present invention is appliedto an actual light-emitting device by using the above describedexpressions 8 and 11 will be described.

In this embodiment, a light-emitting device having a resolution of QVGAof 320×240 and a size of 4 inches will be exemplified.

A pixel size of the 4-inch QVGA light-emitting device is about 84 μm×252μm. When an attempt to obtain definite luminance is made, the intensityof current flowing through an EL element per unit area is determined. Inthis embodiment, it is made 3 mA/cm² per unit area.

Thus, a drain current Id of a current controlling TFT included in eachof pixels is expressed by the following expression 22.

[Expression 22]

The above expression indicates a value of the drain current Id of thecurrent controlling TFT when the opening ratio of the light-emittingdevice is 100%. Actually, in almost all cases, the opening ratio of thelight-emitting device is not 100%. As the opening ratio of thelight-emitting device is lowered, the value of an actually requireddrain current Id becomes large. For example, when the opening ratio ofthe light-emitting device of this embodiment is 30%, the value of theactually required drain current Id is obtained by the followingexpression 23.

[Expression 23]

When the mobility of the current controlling TFT of the light-emittingdevice used in this embodiment is μ=100 (m²/V·sec), and the capacitancevalue of gate capacitance is Co=3×10⁻⁸ (F/cm²), a constant A is obtainedfrom expression 24.

[Expression 24]

A difference in emission luminance of the respective pixels is made tobe restricted to a range of, for example, ±5%. When the gate voltageVgs_((max)) immediately before the TFT is broken is made 25 V, and thevalue of the threshold voltage Vth is made 0 V, the followingexpressions 25 and 26 are obtained from the expressions 8 and 11.

[Expression 25]

[Expression 26]

In the light-emitting device of the present invention, the values ofΔVth and W/L are determined in the range where the above expression 25or 26 is satisfied, and the fluctuation of the drain current Id can besuppressed to the range of ±5%.

For example, in the case where the value of the ratio W/L of the channelwidth W to the channel length L is fixed to 7.5 by a problem of design,when W/L=1/7.5 is substituted in the expression 25, the followingexpression 27 is obtained.

[Expression 27]

If the fluctuation ΔVth of the threshold voltage is determined so thatthe expression 27 is satisfied, the fluctuation of the drain current Idcan be suppressed to the range of ±5%.

Besides, for example, it is assumed that the fluctuation ΔVth of thethreshold voltage is fixed by a fabricating process of TFTs, andΔVth=0.1 V. When ΔVth=0.1 V is substituted in the expression 26, thefollowing expression 28 is obtained.

[Expression 28]

If the ratio W/L of the channel length L and the channel width W isdetermined so that the expression 28 is satisfied, the fluctuation ofthe drain current Id can be suppressed to the range of ±5%.

By the above structure, in the light-emitting device of the presentinvention, the number of thin film transistors provided in each of thepixels is made two to prevent a drop in the opening ratio, and itbecomes possible to suppress uneven luminance due to fluctuation in thethreshold voltage of the current controlling TFTs included in therespective pixels.

Note that in this embodiment, although the description has been given ofthe example in which the fluctuation of the drain current Id issuppressed to the range of ±5%, the present invention is not limited tothis numerical value.

Embodiment 2

A driving method of a light-emitting device of the present inventionwill be described with reference to FIGS. 1 to 3.

FIG. 1 is a top view of a light-emitting device of the presentinvention. Reference numeral 101 designates a source signal line drivingcircuit; 102, a gate signal line driving circuit; and 103, a pixelportion. In this embodiment, although one source signal line drivingcircuit and one gate signal line driving circuit are provided, thepresent invention is not limited to this structure. Two source signalline driving circuits may be provided, or two gate signal line drivingcircuits may be provided.

The source signal line driving circuit 101 includes a shift register101_1, a level shift 101_2, and a sampling circuit 101_3. Note that thelevel shift 101_2 has only to be used as the need arises, and it may notbe always used. Besides, although this embodiment is made to have such astructure that the level shift 101_2 is provided between the shiftregister 101_1 and the sampling circuit 101_3, the present invention isnot limited to this structure. A structure may be such that the levelshift 101_2 is incorporated in the shift register 101_1.

In the pixel portion 103, a source signal line 104 connected to thesource signal line driving circuit 101 intersects with a gate signalline 106 connected to the gate signal line driving circuit 102. A powersupply line 105 is connected to a power source so that it is kept aconstant potential (power source potential).

The gate signal line driving circuit 102 includes a shift register and abuffer (both are not shown). It may include a level shift.

A clock signal (CLK) as a panel control signal, and a start pulse (SP)are inputted to the shift register 101_1. A sampling signal for samplinga video signal is outputted from the shift register 101_1. The outputtedsampling signal is inputted to the level shift 101_2, and is outputtedafter the amplitude of its potential becomes high.

The sampling signal outputted from the level shift 101_2 is inputted tothe sampling circuit 101_3. At the same time, the video signal isinputted to the sampling circuit 101_3 through a video signal line (notshown).

In the sampling circuit 101_3, the inputted video signal is sampled bythe sampling signal, and is inputted to the source signal line 104.

FIG. 2 shows a structure of the pixel portion 103 of the light-emittingdevice shown in FIG. 1. A gate signal line (106_1 to 106 _(—) y) throughwhich a selection signal from the gate signal line driving circuit 102is inputted is connected to a gate electrode of a switching TFT 107included in each of pixels. Besides, one of a source region and a drainregion of the switching TFT 107 included in each of the pixels isconnected to a source signal line (104_1 to 104 _(—) x) through whichthe video signal is inputted, and the other is connected to a gateelectrode of a current controlling TFT 108 included in each of thepixels and a capacitor 110 included in each of the pixels, respectively.

A source region of the current controlling TFT 108 included in each ofthe pixels is connected to a power supply line (105_1 to 105 _(—) x) anda drain region is connected to an anode or a cathode of an EL element109. The power supply line (105_1 to 105 _(—) x) is connected to thecapacitor 110 included in each of the pixels. Note that in thisembodiment, although the structure including the capacitor 110 is shown,the capacitor 110 may not be necessarily provided.

The EL element 109 includes an anode, a cathode, and an EL layerprovided between the anode and the cathode. In the case where the anodeof the EL element 109 is connected to the drain region of the currentcontrolling TFT 108, the anode of the EL element 109 becomes a pixelelectrode, and the cathode becomes a counter electrode. On the contrary,in the case where the cathode of the EL element 109 is connected to thedrain region of the current controlling TFT 108, the anode of the ELelement 109 becomes a counter electrode, and the cathode becomes a pixelelectrode.

FIG. 3 shows a timing chart in the case where the light-emitting deviceshown in FIGS. 1 and 2 is driven by an analog system. A period from apoint when one gate signal line is selected to a point when another gatesignal line is next selected is called one line period (L). Note that inthe present specification, that a gate signal line is selected meansthat a selection signal having such a potential that a switching TFTcomes to have an on state is inputted to the gate signal line.

A period from a point when one image is displayed to a point when a nextimage is displayed corresponds to one frame period (F). In the case ofthe light-emitting device shown in FIG. 2, since there are y gate signallines 104, y line periods (L1 to Ly) are provided in one frame period.

First, the potential (power source potential) of the power supply line(105_1 to 105 _(—) x) is kept constant. The potential of the counterelectrode is also kept constant. The potential of the counter electrodehas a potential difference from the power source potential to such adegree that the EL element emits light when the power source potentialis applied to the pixel electrode of the EL element.

In a first line period (L1), the gate signal line 106_1 is selected bythe selection signal inputted from the gate signal line driving circuit102 through the gate signal line 106_1, and all the switching TFTs 107connected to the gate signal line 106_1 come to have on states. Then,video signals are sequentially inputted to the source signal lines(104_1 to 104 _(—) x) from the source signal line driving circuit 101.The video signals inputted to the source signal lines (104_1 to 104 _(—)x) are respectively inputted to the gate electrodes of the currentcontrolling TFTs 108 through the switching TFTs 107.

The amount of current flowing through a channel forming region of thecurrent controlling TFT 108 is controlled by a gate voltage Vgs of apotential difference between the gate electrode and the source region ofthe current controlling TFT 108. Thus, the potential given to the pixelelectrode of the EL element 109 is determined by the height of thepotential of the video signal inputted to the gate electrode of thecurrent controlling TFT 108. Accordingly, the EL element 109 iscontrolled by the potential of the video signal and emits light.

When the above operation is repeated and the input of the video signalsto the source signal lines (104_1 to 104 _(—) x) is ended, the firstline period (L1) is ended. Note that a combination of a period up to theend of the input of the video signals to the source signal lines (104_1to 104 _(—) x) and a horizontal retrace period may be made one lineperiod. Next, a second line period (L2) is started, the gate signal line106_2 is selected by a selection signal, and video signals aresequentially inputted to the source signal liens (104_1 to 104 _(—) x)similarly to the first line period (L1).

When all the gate signal lines (106_1 to 106 _(—) y) are selected, allline periods (L1 to Ly) are ended. When all the line periods (L1 to Ly)are ended, one frame period is ended. In one frame period, every pixelcauses a display, and one image is formed. Note that a combination ofall the line periods (L1 to Ly) and a vertical retrace period may bemade one frame period.

As described above, the amount of light emission of the EL element iscontrolled by the potential of the video signal, and a gradation displayis carried out by the control of the amount of light emission.

Embodiment 3

In this embodiment, a detailed description will be given of a method offabricating a pixel portion and TFTs (n-channel TFT and p-channel TFT)of a driving circuit provided on the periphery of the pixel portion onthe same substrate at the same time.

First, as shown in FIG. 7A, an under film 701 made of an insulating filmsuch as a silicon oxide film, a silicon nitride film, or a siliconnitride oxide film is formed on a substrate 700 made of glass such asbarium borosilicate glass or alumino borosilicate glass, typified by#7059 glass or #1737 glass of Corning Inc. For example, a siliconnitride oxide film 701 a fabricated from SiH₄, NH₃ and N₂O by a plasmaCVD method is formed to a thickness of 10 to 200 nm (preferably 50 to100 nm), and a hydrogenated silicon nitride oxide film 701 b similarlyfabricated from SiH₄ and N₂O is formed to a thickness of 50 to 200 nm(preferably 100 to 150 nm) to form a laminate. In this embodiment,although the under film 701 is shown as the two-layer structure, thefilm may be formed of a single layer film of the foregoing insulatingfilm or as a laminate structure of more than two layers.

Next, a semiconductor film (amorphous semiconductor film) 702 having athickness of 20 to 150 nm (preferably 30 to 80 nm) and an amorphousstructure is formed by a well-known method such as a plasma CVD methodor a sputtering method. In this embodiment, an amorphous silicon filmwas formed to a thickness of 55 nm by the plasma CVD method. Thesemiconductor film having the amorphous structure includes an amorphoussemiconductor film and a microcrystalline semiconductor film, and acompound semiconductor film having an amorphous structure, such as anamorphous silicon germanium film, may also be applied. Since the underfilm 701 and the amorphous silicon film 702 can be formed by the samefilm forming method, both may be continuously formed. When the underfilm is not exposed to the air after its formation, it becomes possibleto prevent contamination of its surface, and the fluctuation ofcharacteristics of fabricated TFTs and the change of threshold voltagecan be decreased (FIG. 7A).

Next, a crystalline semiconductor film is formed by a thermalcrystallization method using a catalytic element. In the case where thecatalytic element is used, it is desirable to use a technique disclosedin Japanese Patent Laid-Open No. Hei 7-130652 or No. Hei 8-78329.

First, a silicon oxide film having a thickness of 150 nm was formed onthe amorphous semiconductor film 702, and patterning was carried out toform masks 703 to 705. The silicon oxide film may be continuously formedtogether with the amorphous semiconductor film 702, or may becontinuously formed together with the under film 701 and the amorphoussemiconductor film 702.

Next, a nickel acetate salt solution containing nickel of 10 ppm interms of weight was coated. By this, a nickel containing layer 706 wasformed, and the nickel containing layer 706 was brought into contactwith the amorphous semiconductor film 702 only at bottom portions ofopening portions 707 and 708 (FIG. 7B).

Next, a heat treatment at 500 to 650° C. for 4 to 24 hours, for example,at 570° C. for 14 hours was carried out to form a crystallinesemiconductor film 709. In this crystallization process, a portion ofthe amorphous semiconductor film 702 with which nickel is in contact isfirst crystallized, and crystallization proceeds in the horizontaldirection from that. The crystalline semiconductor film 709 formed inthis way is made of an aggregate of rod-like or needle-like crystals,and the respective crystals grow with certain specific directionalitywhen they are macroscopically seen, so that there is a merit thatcrystallinity is uniform (FIG. 7B).

Note that in the above two techniques, as a usable catalytic element, inaddition to nickel (Ni), an element such as germanium (Ge), iron (Fe),palladium (Pd), tin (Sn), lead (Pb), cobalt (Co), platinum (Pt), copper(Cu), or gold (Au) may be used.

Next, phosphorus was doped so that regions 710 and 711 added withphosphorus were provided in regions where the crystalline semiconductorfilm 709 was exposed at the opening portions 707 and 708.

In this state, when a heat treatment at 550 to 800° C. for 5 to 24hours, for example, at 600° C. for 12 hours was carried out in anitrogen atmosphere, the regions 710 and 711 where phosphorus was addedinto the crystalline semiconductor film 709 function as gettering cites,so that it was possible to make the catalytic element remaining in thecrystalline semiconductor film 709 segregate into the regions 710 and711 added with phosphorus (FIG. 7C).

Then, the masks 703 to 705 and the regions 710 and 711 where phosphoruswas added were removed by etching, and patterning was carried out, sothat it was possible to obtain island-like semiconductor films 712 to715 where the concentration of the catalytic element used in the step ofcrystallization was reduced to 1×10¹⁷ arms/cm³ or less.

Note that in this embodiment, although crystallization of the amorphoussemiconductor film 702 was carried out by using the catalytic element,the present invention is not limited to this method, but a well-knowncrystallization technique can be used. As the well-known crystallizationtechnique, for example, a heat crystallization method using anelectronic furnace, a laser annealing crystallization method using laserlight, and a lamp annealing crystallization method using infrared lightcan be named as a well-known crystallization method.

In order to fabricate the crystalline semiconductor film by the lasercrystallization method, a pulse oscillation type or continuous-waveexcimer laser, YAG laser, or YVO₄ laser is used. In the case where suchlaser is used, it is appropriate that there is used a method in whichlaser light radiated from a laser oscillator is collected by an opticalsystem into a linear beam and is irradiated to the amorphoussemiconductor film. Although the condition of crystallization should beproperly selected by an operator, in the case where the excimer laser isused, a pulse oscillation frequency is made 300 Hz, and a laser energydensity is made 100 to 400 mJ/cm² (typically 200 to 300 mJ/cm²). In thecase where the YAG laser is used, it is appropriate that the secondharmonic is used, a pulse oscillation frequency is made 30 to 300 Hz,and a laser energy density is made 300 to 600 mJ/cm² (for example, 350to 500 mJ/cm²). Then, laser light collected into a linear shape with awidth of 100 to 1000 μm, for example, 400 μm is irradiated to the wholesurface of the substrate, and an overlapping ratio (overlap ratio) ofthe linear laser light at this time is made 50 to 90%.

Besides, before the step of crystallization, although depending on thehydrogen content of the amorphous semiconductor film, crystallizationmay be carried out after a heat treatment at 400 to 500° C. for about 1hour is carried out to make the hydrogen content 5 atom % or less. Whenthe amorphous semiconductor film is crystallized, since atoms arerearranged and the film becomes dense, the thickness of the fabricatedcrystalline semiconductor film was decreased by about 1 to 15% from theinitial thickness of the amorphous semiconductor film.

The island-like semiconductor layers 712 to 715 are formed to athickness of 25 to 80 nm (preferably 30 to 60 nm).

Next, a first shape gate insulating film 716 covering the island-likesemiconductor layers 712 to 715 is formed. The first shape gateinsulating film 716 is formed of an insulating film having a thicknessof 40 to 150 nm and containing silicon by using a plasma CVD method or asputtering method. In this embodiment, this film is formed of a siliconnitride oxide film having a thickness of 120 nm. Of course, the gateinsulating film is not limited to such silicon nitride oxide film, butanother insulating film containing silicon may be used as a single layeror a laminate structure. For example, in the case where a silicon oxidefilm is used, TEOS (Tetraethyl Orthosilicate) and O₂ are mixed with eachother by the plasma CVD method, a reaction pressure is made 40 Pa, asubstrate temperature is made 300 to 400° C., and discharge is made at ahigh frequency (13.56 MHz) power density 0.5 to 0.8 W/cm², so that thefilm can be formed. Thereafter, the silicon oxide film fabricated inthis way is subjected to heat annealing at 400 to 500° C. so thatexcellent characteristics as the gate insulating film can be obtained(FIG. 7D).

Then, a first conductive film 718 and a second conductive film 719 forforming a gate electrode are formed on the first shape gate insulatingfilm 716. In this embodiment, the first conductive film 718 is formed ofTa (tantalum) and to a thickness of 50 to 100 nm, and the secondconductive film 719 is formed of W (tungsten) and to a thickness of 100to 300 nm (FIG. 8A).

The Ta film is formed by a sputtering method, and a target of Ta issputtered by Ar. In this case, when a suitable amount of Xe or Kr isadded to Ar, it is possible to relieve internal stress of the Ta filmand to prevent peeling of the film. Although the resistivity of aα-phase Ta film is about 20 μΩcm and can be used as the gate electrode,the resistivity of a β-phase Ta film is about 180 μΩcm and is unsuitablefor the gate electrode. In order to form the α-phase Ta film, iftantalum nitride having crystal structure close to the α phase of Ta isformed to a thickness of about 10 to 50 nm as an under layer of Ta, theα-phase Ta film can be easily obtained.

In the case where the W film is formed, the film is formed by thesputtering method using W as a target. In addition to this, the film canalso be formed by a thermal CVD method using tungsten hexafluoride(WF₆). In any case, in order to use the film as the gate electrode, itis necessary to lower the resistance, and it is desirable that theresistivity of the W film is made 20 μΩcm or less. Although theresistivity of the W film can be lowered by enlarging crystal grains, inthe case where a lot of impurity elements of oxygen or the like exist inW, crystallization is hindered and the resistivity becomes high. Fromthis, in the case of the sputtering method, a W target of purity99.9999% or 99.99% is used, and further, the W film is formed whilearrangements are thoroughly made to prevent an impurity from mixing froma vapor phase at the film formation, so that a resistivity of 9 to 20μΩcm can be realized.

Note that in this embodiment, although the first conductive film 718 ismade of Ta, and the second conductive film 719 is made of W, the presentinvention is not particularly limited, and either film may be formed ofan element selected from Ta, W, Ti, Mo, Al, and Cu, or an alloy materialor a compound material containing the above element as its mainingredient. Besides, a semiconductor film typified by a polycrystallinesilicon film doped with an impurity element such as phosphorus may beused. As examples of combinations other than this embodiment, it ispreferable to form the film by a combination in which the firstconductive film is formed of tantalum nitride (TaN) and the secondconductive film is formed of W, a combination in which the firstconductive film is formed of tantalum nitride (TaN) and the secondconductive film is formed of Al, or a combination in which the firstconductive film is formed of tantalum nitride (TaN) and the secondconductive film is formed of Cu.

Next, resist masks 720 to 726 are formed, and a first etching treatmentfor forming electrodes and wiring lines is carried out. In thisembodiment, an ICP (Inductively Coupled Plasma) etching method is used,in which CF₄ and Cl₂ are mixed in an etching gas, and an RF (13.56 MHz)power of 500 W is applied to a coil type electrode under a pressure of 1Pa to generate plasma. An RF (13.56 MHz) power of 100 W is also appliedto the side of the substrate (sample stage) and a substantially negativeself bias voltage is applied. In the case where CF₄ and Cl₂ are mixedwith each other, both the W film and the Ta film are etched to the samedegree.

Under the above etching condition, by making the shapes of the resistmasks suitable, end portions of a first conductive layer and a secondconductive layer become taper-shaped by the effect of the bias voltageapplied to the substrate side. The angle of the taper portion becomes 15to 45°. In order to carry out the etching without leaving a residue onthe gate insulating film, it is appropriate that an etching time isincreased at a rate of about 10 to 20%. Since the selection ratio of thesilicon nitride oxide film to the W film is 2 to 4 (typically 3), asurface on which the silicon nitride oxide film is exposed is etched byan over etching treatment by about 20 to 50 nm. In this way, first shapeconductive layers 727 to 733 made of first conductive layers and secondconductive layers (first shape first conductive layers 722 a to 733 aand first shape second conductive layers 722 b to 733 b) are formed bythe first etching treatment. Reference numeral 750 designates a secondshape gate insulating film, and regions which are not covered with thefirst shape conductive layers 727 to 733 are etched by about 20 to 50 nmso that thinned regions are formed (FIG. 83).

Then, a first doping treatment is carried out to add an impurity elementto give an n type. Doping may be carried out by an ion doping method oran ion injecting method. The condition of the ion doping method is thata dosage is 1×10¹³ to 5×10¹⁴ atoms/cm², and an acceleration voltage is60 to 100 keV. As the impurity element to give the n type, although anelement belonging to group 15, typically phosphorus (P) or arsenic (As)is used, phosphorus is used here. In this case, The first shapeconductive layers 728, 729, 731, and 733 become masks to the impurityelement to give the n type, and first impurity regions 734 to 737 areformed in a self aligning manner. The impurity element to give the ntype in the concentration range of 1×10²⁰ to 1×10²¹ atoms/cm³ is addedto the first impurity regions 734 to 737 (FIG. 8B).

Next, as shown in FIG. 8C, a second etching treatment is carried out.The ICP etching method is similarly used, in which CF₄, Cl₂ and O₂ aremixed in an etching gas, and an RF power (13.56 MHz) of 500 W is appliedto a coil type electrode under a pressure of 1 Pa to generate plasma. AnRF (13.56 MHz) power of 50 W is applied to the side of the substrate(sample stage) and a low self bias voltage as compared with the firstetching treatment is applied. The W film is anisotropically etched underthe condition like this, and the Ta film as the first conductive layersis anisotropically etched at an etching rate lower than that to formsecond shape conductive layers 738 to 744 (second shape first conductiveLayers 738 a to 744 a and second shape second conductive layers 738 b to744 b). Reference numeral 745 designates a third shape gate insulatingfilm, and regions which are not covered with the second shape conductivelayers 738 to 744 are further etched by about 20 to 50 nm so thatthinned regions are formed.

An etching reaction of the W film or the Ta film by the mixture gas ofCF₄ and Cl₂ can be guessed from a generated radical or ion species andthe vapor pressure of a reaction product. When the vapor pressures offluoride and chloride of W and Ta are compared with each other, WF₆ asfluoride of W is extremely high, and other WCl₅, TaF₅, and TaCl₅ havealmost equal vapor pressures. Thus, in the mixture gas of CF₄ and Cl₂,both the W film and the Ta film are etched. However, when a suitableamount of O₂ is added to this mixture gas, CF₄ and O₂ react with eachother to form CO and F, and a large number of F radicals or F ions aregenerated. As a result, an etching rate of the W film having the highvapor pressure of fluoride is increased. On the other hand, with respectto Ta, even if F is increased, an increase of the etching rate isrelatively small. Besides, since Ta is easily oxidized as compared withW, the surface of Ta is oxidized by addition of O₂. Since the oxide ofTa does not react with fluorine or chlorine, the etching rate of the Tafilm is further decreased. Accordingly, it becomes possible to make adifference between the etching rates of the W film and the Ta film, andit becomes possible to make the etching rate of the W film higher thanthat of the Ta film.

Then, as shown in FIG. 8D, a second doping treatment is carried out. Inthis case, a dosage is made lower than that of the first dopingtreatment and under the condition of a high acceleration voltage, animpurity element to give the n type is doped. For example, anacceleration voltage is made 70 to 120 keV, and the treatment is carriedout at a dosage of 1×10¹³ atoms/cm², so that new impurity regions areformed inside of the first impurity regions formed into the island-likesemiconductor layers in FIG. 8B. Doping is carried out in such a mannerthat the second shape conductive layers 739, 740, 742 and 744 are usedas masks to the impurity element and the impurity element is added alsoto the regions under the second conductive layers 739 a, 740 a, 742 aand 744 a. In this way, third impurity regions 746 b to 749 boverlapping with the second conductive layers 739 a, 740 a, 742 a and744 a, and second impurity regions 746 a to 749 a between the firstimpurity regions and the third impurity regions are formed. The impurityelement to give the n type is made to have a concentration of 1×10¹⁷ to1×10¹⁹ atoms/cm³ in the second impurity regions, and a concentration of1×10¹⁶ to 1×10¹⁸ atoms/cm³ in the third impurity regions.

Then, as shown in FIG. 9A, fourth impurity regions 753 a and 754 a,fifth impurity regions 753 b and 754 b, sixth impurity regions 753 c and754 c having a conductivity type opposite to the former conductivitytype are formed in the island-like semiconductor layers 713 and 715forming p-channel TFTs. The second conductive layers 740 and 744 areused as masks to an impurity element, and the impurity regions areformed in a self aligning manner. At this time, the whole surfaces ofthe island-like semiconductor layers 712 and 714 forming n-channel TFTsare covered with resist masks 751 and 752. Phosphorus is added to theimpurity regions 753 a, 753 b and 753 c at different concentrations,respectively, and phosphorus is added to the impurity regions 754 a, 754b and 754 c at different concentrations, respectively. The regions areformed by an ion doping method using diborane (B₂H₆) and the impurityconcentration is made 2×10²⁰ to 2×10²¹ atoms/cm³ in any of the regions.

By the steps up to this, the impurity regions are formed in therespective island-like semiconductor regions. The second shape secondconductive layers 739, 740, 742, and 744 overlapping with theisland-like semiconductor layers function as gate electrodes. The layer741 functions as an island-like source signal line, the layer 738functions as a wiring line, and the layer 743 functions as a gate signalline.

A step of activating the impurity elements added in the respectiveisland-like semiconductor layers for the purpose of controlling theconductivity type in this way, as shown in FIG. 9B, is carried out. Thisstep is carried out by a thermal annealing method using a furnaceannealing oven. In addition, a laser annealing method or a rapid thermalannealing method (RTA method) can be applied. The thermal annealingmethod is carried out in a nitrogen atmosphere having an oxygen contentof 1 ppm or less, preferably 0.1 ppm or less and at 400 to 700° C.,typically 500 to 600° C. In this embodiment, a heat treatment at 500° C.for 4 hours is carried out. However, in the case where a wiring materialused for the second conductive layers 738 to 744 is weak to heat, it ispreferable that the activation is carried out after an interlayerinsulating film (containing silicon as its main ingredient) is formed toprotect the wiring line or the like.

Further, a heat treatment at 300 to 450° C. for 1 to 12 hours is carriedout in an atmosphere containing hydrogen of 3 to 100%, so that a step ofhydrogenating the island-like semiconductor layers is carried out. Thisstep is a step of terminating dangling bonds in the semiconductor layerby thermally excited hydrogen. As another means for hydrogenation,plasma hydrogenation (using hydrogen excited by plasma) may be carriedout.

Next, a first interlayer insulating film 755 having a thickness of 100to 200 nm is formed from a silicon nitride oxide film. A secondinterlayer insulating film 756 made of an organic insulator material isformed thereon. Next, an etching step for forming contact holes iscarried out.

Then, in a driving circuit 806, source wiring lines 757 and 758 incontact with source regions of the island-like semiconductor layers anddrain wiring lines 759 and 760 in contact with drain regions are formed.In a pixel portion 807, a connection electrode 761, source wiring lines761 and 762, and drain wiring lines 763 and 764 are formed (FIG. 9C). Bythis connection electrode 761, the island-like source signal line 741 iselectrically connected to the switching TFT 804.

In the manner as described above, the driving circuit 806 including anre-channel TFT 801 and a p-channel TFT 802 and the pixel portion 807including a switching TFT 804 and a current controlling TFT 805 can beformed on the same substrate. In the present specification, such asubstrate is called an active matrix substrate for convenience.

The n-channel TFT 801 of the driving circuit 806 includes a channelforming region 788, the third impurity region 746 b (GOLD region)overlapping with the second shape second conductive layer 739 formingthe gate electrode, the second impurity region 746 a (LDD region) incontact with the third impurity region 746 b, and the first impurityregion 734 functioning as a source region or a drain region. Thep-channel TFT 802 includes a channel forming region 789, the fourthimpurity region 753 c overlapping with the second conductive layer 740forming the gate electrode, the fifth impurity region 753 b in contactwith the fourth impurity region 753 c, and the sixth impurity region 753a functioning as a source region or a drain region.

The switching TFT 804 of the pixel portion includes a channel formingregion 790, the third impurity region 748 b (GOLD region) overlappingwith the second shape second conductive layer 742 forming the gateelectrode, the second impurity region 748 a (LDD region) in contact withthe third impurity region 748 b, and the first impurity region 736functioning as a source region or a drain region. The currentcontrolling TFT 805 includes a channel forming region 791, the fourthimpurity region 754 c overlapping with the second shape secondconductive layer 744 forming the gate electrode, the fifth impurityregion 754 b in contact with the fourth impurity region 754 c, and thesixth impurity region 754 a functioning as a source region or a drainregion.

Next, as shown in FIG. 10A, a first passivation film 766 is formed to athickness of 50 to 500 nm (typically 200 to 300 nm). In this embodiment,as the first passivation film 766, a silicon nitride oxide film having athickness of 300 nm is used. A silicon nitride film may be substitutedfor this. Note that it is effective to carry out a plasma treatmentusing a gas containing hydrogen, such as H₂ or NH₃, before the siliconnitride oxide film is formed. Hydrogen excited by this pretreatment issupplied to the second interlayer insulating film 756, and the filmquality of the first passivation film 766 is improved by carrying out aheat treatment. At the same time, since hydrogen added to the secondinterlayer insulating film 756 is diffused to the lower layer side, theactive layer can be effectively hydrogenated.

Next, a third interlayer insulating film 767 made of an organic resin isformed. As the organic resin, polyimide, polyamide, acryl, BCB(benzocyclobutene) or the like can be used. Especially, since the thirdinterlayer insulating film 767 has rather the meaning of flattening,acryl excellent in flatness is desirable. In this embodiment, an acrylfilm is formed to such a thickness that stepped portions formed by theTFTs can be adequately flattened. It is appropriate that the thicknessis preferably made 1 to 5 μm (more preferably 2 to 4 μm) (FIG. 10A).

Next, a contact hole reaching the drain wiring line 764 is formed in thethird interlayer insulating film 767 and the first passivation film 766,and a pixel electrode 768 is formed. In this embodiment, an indium-tinoxide (ITO) film is formed to a thickness of 110 nm, and patterning iscarried out to form the pixel electrode 768. Besides, a transparentelectrode in which zinc oxide (ZnO) of 2 to 20% is mixed with indiumoxide may be used. This pixel electrode 768 corresponds to an anode ofan EL element.

Next, an organic resin film is formed on the pixel electrode 768 and thethird interlayer insulating film 767, and the organic resin film ispatterned, so that a bank 769 and a flattening portion 770 are formed.In this embodiment, as the organic resin film, an acryl film or apolyimide film having a thickness of 1 to 2 μm was used.

The bank 769 is formed into a stripe shape between a pixel and a pixelto separate light-emitting layers or EL layers of adjacent pixels. Inthis embodiment, although the bank 769 is formed along the source wiringline 741, it may be formed along the gate wiring line 743. Note that apigment or like may be added to the resin material forming the bank 769so that the bank 769 is used as a light shielding film.

The flattening portion 770 is provided on a portion where the pixelelectrode 768 is connected with the drain wiring line 764 of the currentcontrolling TFT 805. Since there is a case where the connection of thepixel electrode 768 with the drain wiring line 764 is cut off by astepped portion of the contact hole, it is desirable to make flatteningby providing the flattening portion 770 in order to prevent poor lightemission of an EL layer 771 formed later. Note that the bank 769 and theflattening portion 770 may not be formed to the same thickness, and canbe suitably set in accordance with the thickness of the later formed ELlayer 771.

Next, the EL layer 771 and a cathode (MgAg electrode) 722 arecontinuously formed by using a vacuum evaporation method withoutexposing to the air. Note that it is appropriate that the thickness ofthe EL layer 771 is made 80 to 200 nm (typically 100 to 120 nm), and thethickness of the cathode 772 is made 180 to 300 nm (typically 200 to 250nm). Note that in this embodiment, although only one pixel is shown, atthis time, an EL layer emitting red light, an EL layer emitting greenlight, and an EL layer emitting blue light are formed at the same time.

In this step, the EL layer 771 is sequentially formed for a pixelcorresponding to red, a pixel corresponding to green, and a pixelcorresponding to blue. However, since the EL layer 771 has poorresistance against a solution, the layer must be formed individually forrespective colors without using a photolithography technique. Then, itis preferable that portions other than a desired pixel are concealed byusing metal masks, and the EL layer 771 is selectively formed on only anecessary portion.

That is, first, a mask for concealing all portion other than the pixelcorresponding to red is set, and the EL layer emitting red light isselectively formed using the mask. Next, a mask for concealing allportion other than the pixel corresponding to green is set, and the ELlayer emitting green light is selectively formed using the mask. Next,similarly, a mask for concealing all portion other than the pixelcorresponding to blue is set, and the EL layer emitting blue light isselectively formed using the mask. Note that here, although therecitation is such that different masks are used for the respectivepixels, the same mask may be commonly used. Besides, it is preferablethat the treatment is carried out without breaking a vacuum until the ELlayers are formed for all pixels.

Note that in this embodiment, although the EL layer 771 is made to havea single layer structure of only a light-emitting layer, the EL layermay includes a hole transporting layer, a hole injecting layer, anelectron transporting layer, an electron injecting layer, or the like inaddition to the light-emitting layer. Like this, various examples havebeen already reported with respect to the combination, and any structureof those may be used. As the EL layer 771, a well-known material can beused. As the well-known material, in view of EL driving voltage, it ispreferable to use an organic material.

Next, the cathode 772 is formed. Although this embodiment shows anexample in which a MgAg electrode is used as a cathode of an EL element,another well-known material can be used.

In this way, an active matrix substrate having a structure as shown inFIG. 10B is completed. Note that it is effective that after the bank 769and the flattening portion 770 are formed, steps up to the formation ofthe cathode 772 are continuously carried out by using a thin filmforming apparatus of a multichamber system (or an inline system) withoutopening to the air.

In this embodiment, the switching TFT 804 is made to have a double gatestructure, and by making the double gate structure, there is obtained astructure in which two TFTs are substantially connected in series witheach other, and there is a merit that an off current value can bedecreased. Although this embodiment adopts the double gate structure, asingle gate structure may be adopted, or a triple gate structure or amultigate structure having more gates may be adopted.

Note that actually, when the state of FIG. 10B is completed, it ispreferable to make packaging (sealing) by a protective film (laminatefilm, ultraviolet ray curing resin film, etc.), which has highairtightness and hardly causes degassing, or a translucent sealingmember so as to prevent exposure to the outer air. At that rime, if theinside of the sealing member is made an inert atmosphere or ahygroscopic material (for example, barium oxide) is disposed in theinside, the reliability of the EL element is improved.

After the airtightness is raised by the treatment such as packaging, aconnector (Flexible Printed Circuit: FPC) for connecting a terminalextended from the element or the circuit formed on the substrate to anexternal signal terminal is attached so that a product is completed.

Embodiment 4

In this embodiment, an example in which a light-emitting device isfabricated by using the present invention will be described. FIG. 11A isa top view of the light-emitting device of the present invention, andFIG. 11B is a sectional view thereof.

In FIG. 11A, reference numeral 4001 designates a substrate; 4002, apixel portion; 4003, a source signal line driving circuit; and 4004, agate signal line driving circuit. The respective driving circuits leadto an FPC (Flexible Printed Circuit) 4006 through a wiring line 4005 andare connected to an external instrument.

At this time, a first seal member 4101, a cover member 4102, a fillermember 4103, and a second seal member 4104 are provided so as tosurround the pixel portion 4002, the source signal line driving circuit4003, and the gate signal line driving circuit 4004.

FIG. 11B is a sectional view taken along line A-A′ of FIG. 11A, and adriving TFT (here, an n-channel TFT and a p-channel TFT are shown) 4201included in the source signal line driving circuit 4003 and a currentcontrolling TFT (TFT for controlling current to an EL element) 4202included in the pixel portion 4002 are formed on the substrate 4001.Note that in FIG. 11B, a switching TFT is not shown for simplificationof the explanation.

In this embodiment, the driving TFT 4201 and the current controlling TFT4202 are formed by using a well-known fabricating method. Besides, aholding capacitance (not shown) connected to a gate electrode of thecurrent controlling TFT 4202 is provided in the pixel portion 4002.

An interlayer insulating film (flattening film) 4301 made of a resinmaterial is formed on the driving TFT 4201 and the switching TFT 4202,and a pixel electrode (anode) 4302 electrically connected to a drainregion of the pixel TFT 4202 is formed thereon. As the pixel electrode4302, a transparent conductive film having a large work function isused. As the transparent conductive film, a compound of indium oxide andtin oxide, a compound of indium oxide and zinc oxide, zinc oxide, tinoxide, or indium oxide can be used. The transparent conductive filmadded with gallium may be used.

Then, an insulating film 4303 is formed on the pixel electrode 4302, andan opening portion is formed in the insulating film 4303 on the pixelelectrode 4302. At this opening portion, an EL (electroluminescence)layer 4304 is formed on the pixel electrode 4302. As the EL layer 4304,a well-known organic EL material or an inorganic EL material can beused. Although the organic EL material includes a low molecular(monomer) material and a high molecular (polymer) material, either maybe used.

As a method of forming the EL layer 4304, a well-known evaporationtechnique or coating technique may be used. The structure of the ELlayer may be made a laminate structure by freely combining a holeinjecting layer, a hole transporting layer, a light-emitting layer, anelectron transporting layer, and an electron injecting layer or a singlelayer structure.

A cathode 4305 made of a conductive film (typically, a conductive filmcontaining aluminum, copper or silver as its main ingredient, or alaminate film of those and another conductive film) having a lightshielding property is formed on the EL layer 4304. It is desirable thatmoisture and oxygen existing at the interface between the cathode 4305and the EL layer 4304 are removed to the utmost degree. Accordingly, itis necessary to make such contrivance that both are continuously formedin vacuum, or the EL layer 4304 is formed in a nitrogen or rare gasatmosphere, and the cathode 4305 is formed while the layer is not putinto contact with oxygen and moisture. In this embodiment, a filmforming apparatus of a multichamber system (cluster tool system) is usedso that the film formation as described above is made possible.

The cathode 4305 is electrically connected to the wiring line 4005 in aregion designated by 4306. The wiring line 4005 is a wiring line forgiving a predetermined voltage to the cathode 4305, and is electricallyconnected to the FPC 4006 through an anisotropic conductive film 4307.

In the manner as described above, an EL element constituted by the pixelelectrode (anode) 4302, the EL layer 4304, and the cathode 4305 isformed. This EL element is surrounded by the cover member 4102 bonded tothe substrate 4001 by the first seal member 4101 and the second sealmember 4104 and is sealed by the filler member 4103.

As the cover member 4102, a glass member, a metal member (typically astainless member), a ceramic member, or a plastic member (including aplastic film as well) can be used. As the plastic member, an FRP(Fiberglass-Reinforced Plastics) plate, a PVF (polyvinyl fluoride) film,a Mylar film, a polyester film, or an acrylic resin film can be used.Besides, a sheet having a structure in which an aluminum foil issandwiched between PVF films or Mylar films can also be used.

However, in the case where the radiation of light from the EL element isdirected toward the side of the cover member, the cover member must betransparent. In that case, a transparent material such as a glass plate,a plastic plate, a polyester film or an acryl film is used.

As the filler member 4103, an ultraviolet ray curing resin or athermosetting resin can be used, and PVC (polyvinyl chloride), acryl,polyimide, epoxy resin, silicone resin, PVB (polyvinyl butyral) or EVA(ethylene-vinyl acetate) can be used. If a hygroscopic material(preferably barium oxide) or a material capable of adsorbing oxygen isprovided in the inside of this filler member 4103, deterioration of theEL element can be suppressed.

A space may be included in the filler member 4103. At this time, if thespacer is formed of barium oxide, it is possible to make the spaceritself have a hygroscopic property. Besides, in the case where thespacer is provided, as a buffer layer for relieving stress from thespacer, it is also effective to provide a resin film on the cathode4305.

The wiring line 4005 is electrically connected to the FPC 4006 throughthe anisotropic conductive film 4307. The wiring line 4005 connected tothe pixel portion 4002, the source signal line driving circuit 4003, andthe gate signal line driving circuit 4004 are electrically connected toan outside instrument through the FPC 4006.

Besides, in this embodiment, a second seal member 4104 is provided so asto cover an exposed portion of the first seal member 4101 and a part ofthe FPC 4006, and a structure to thoroughly shut off the EL element fromthe outer air is adopted.

Embodiment 5

In this embodiment, examples of pixel structures which can be used forthe pixel portion of the light-emitting device set forth in theembodiments 1 to 4 are shown in FIGS. 12A to 12C. In this embodiment,reference numeral 4601 designates a source signal line; 4602, aswitching TFT; 4603, a gate signal line; 4604, a current controllingTFT; 4605, a capacitor; 4606 and 4608, power supply lines; and 4607, anEL element.

FIG. 12A is a circuit diagram in a case where two pixels including thesame gate signal line own the power supply line 4606 jointly. That is,the feature is that the two pixels are formed to be axially symmetricalwith respect to the power supply line 4606. In this case, since thenumber of power supply lines can be decreased, the pixel portion can bemade highly fine.

FIG. 12B is a circuit diagram in a case where the power supply line 4608is provided in parallel with the gate signal line 4603. Note thatalthough FIG. 12B shows the structure in which the power supply line4608 does not overlap with the gate signal line 4603, if both are wiringlines formed in different layers, they can also be formed so as tooverlap with each other through an insulating film. In this case, sincean occupied area can be jointly owned by the power supply line 4608 andthe gate signal line 4603, the pixel portion can be made highly fine.

FIG. 12C has a feature that the power supply line 4608 is provided inparallel with gate signal lines 4603 (4603 a, 4603 b) similarly to thestructure of FIG. 12B, and further, two pixels are formed to be axiallysymmetrical with respect to the power supply line 4608. It is alsoeffective that the power supply line 4608 is provided to overlap withone of the gate signal lines 4603 a and 4603 b. In this case, since thenumber of power supply lines can be decreased, the pixel portion can befurther made highly fine.

Embodiment 6

In this embodiment, a detailed circuit structure of a source signal linedriving circuit of a light-emitting device of the present invention willbe described with reference to FIG. 13.

Reference numeral 1301 designates a shift register; 1302, a level shift;1303, a sampling circuit; 1304, an analog switch; and 1305, a videosignal.

A clock signal (CLK) as a panel control signal, and a start pulse signal(SP) are inputted to the shift register 1301. A sampling signal forsampling a video signal is outputted from the shift register 1301. Theoutputted sampling signal is inputted to the level shift 1302.

The amplitude of the potential of the clock signal inputted to the levelshift 1302 is made large. FIG. 14A is an equivalent circuit diagram ofthe level shift 1302. Reference characters Vin and Vinb designate inputterminals, and Vinb means that a signal having a potential equal to aninversion of a potential of a signal inputted to Vin is inputted.Reference character Vddh designates a voltage of a power source at ahigh voltage side, and Vss designates a voltage of a power source at alow voltage side. Reference character Voutb designates an outputterminal, and the level shift 1302 is designed such that a signalobtained by boosting and inverting a signal inputted to Vin is outputtedfrom Voutb. That is, when Hi is inputted to Vin, a signal correspondingto Vss is outputted from Voutb, and when Lo is inputted, a signalcorresponding to Vddh is outputted from Vout.

Note that in this embodiment, although the level shift having thestructure shown in FIG. 14A is used, the present invention is notlimited to this. In the light-emitting device of the present invention,a level shift having a well-known structure can be used.

The sampling signal outputted from the level shift 1302 is inputted tothe sampling circuit 1303. At the same time the video signal is inputtedto the sampling circuit 1303 through the video signal line 1305.

The sampling circuit 1303 includes the analog switch 1304. FIG. 14B isan equivalent circuit diagram of the analog switch 1304 used in thisembodiment. A voltage of the sampling signal inputted to the samplingcircuit 1303 is applied to a gate electrode of a TFT constituting theanalog switch 1304 of the sampling circuit 1303. By this, a channel isformed in the TFT constituting the analog switch 1304, and a currentflows from a source to a drain. Thus, the video signal is sampled and issupplied to a source of a pixel TFT through a source signal line (S1,S2).

Note that although the analog switch having the structure shown in FIG.14B is used in this embodiment, the present invention is not limited tothis. In the light-emitting device of the present invention, an analogswitch having a well-known structure can be used. Besides, in FIG. 13,although only two signal lines S1 and S2 are shown for simplification ofthe explanation, the number of source signal lines of this embodiment isnot limited to this.

Note that this embodiment can be carried out in combination with allembodiments of the present specification.

Embodiment 7

In this embodiment, an upper view of a pixel of a light-emitting deviceof the present invention is shown in FIG. 15 as an example.

Reference numeral 1501 designates a switching TFT, which includes a gateelectrode 1501_1 and an active layer 1501_2. Reference numeral 1502designates a current controlling TFT, which includes an active layer1502_2 and a gate electrode 1502_2 as a part of a wiring line 1509. Notethat in this embodiment, although the switching TFT 1501 is made to havea double gate structure, and the current controlling TFT 1502 is made tohave a single gate structure, the present invention is not limited tothis constitution. The switching TFT 1501 and the current controllingTFT 1502 may have a single gate structure, or may have a multigatestructure such as a double gate structure or a triple gate structure.

Reference numeral 1503 designates a source signal line; 1504, a powersupply line; and 1505, a gate signal line. The source signal line 1503is connected to the active layer 1501_2 of the switching TFT 1501through a contact hole. The power supply line 1504 is connected to asource region of the active layer 1502_2 of the current controlling TFT1502 through a contact hole. The gate signal line 1505 is connected tothe gate electrode 1501_1 of the switching TFT 1501.

The wiring line 1509 including the gate electrode 1502_1 of the currentcontrolling TFT 1502 overlaps with the power supply line 1504 through aninsulating film in a region indicated by 1511. At this time, a holdingcapacitance (capacitor) is formed in the region indicated by 1511. Theholding capacitance 1511 is formed of a semiconductor film 1510electrically connected to the power supply line 1504, an insulating film(not shown) of the same layer as the gate insulating film, and thewiring line 1509. A capacitance formed of the wiring line 1509, the samelayer (not shown) as the first interlayer insulating film, and the powersupply line 1504 can also be used as a holding capacitance. This holdingcapacitance 1511 functions as a capacitor for holding voltage applied tothe gate electrode 1502_1 of the current controlling TFT 1502. Thesource region of the current controlling TFT 1502 is connected to thepower supply line (power source line) 1504 and a constant voltage isalways applied.

A first passivation film (not shown) is provided on the switching TFT1501 and the current controlling TFT 1502, and a flattening film (thirdinterlayer insulating film) (not shown) made of a resin insulating filmis formed thereon. It is very important to flatten a stepped portion dueto a TFT by using the flattening film. Since a later formed EL layer(not shown) is very thin, there is a case where poor light emissionoccurs due to the existence of the stepped portion. Accordingly, it isdesirable to make flattening before a pixel electrode 1507 is formed sothat the EL layer can be formed on the flattest possible surface.

Reference numeral 1507 designates the pixel electrode (a cathode of anEL element) made of a conductive film having high reflectivity, and iselectrically connected to the drain region of the current controllingTFT 1502 through a contact hole provided in the first passivation filmand the flattening film. As the pixel electrode 1507, it is desirable touse a low resistance conductive film such as an aluminum alloy film, acopper alloy film or a silver alloy film, or a laminate film of those.Of course, a laminate structure with another conductive film may beadopted.

Next, an organic resin film is formed on the pixel electrode 1507 andthe flattening film, and the organic resin film is patterned, so that abank 1506 is formed. The bank 1506 is provided to separatelight-emitting layers or EL layers of adjacent pixels. Thelight-emitting layer (not shown) is formed in a groove (corresponding toa pixel) formed by the bank 1506. Note that in FIG. 15, although thebank is partially omitted in order to clarify the position of theholding capacitance 1511, it is provided between pixels so as to coverpart of the power supply line 1503 and the source signal line 1504.Besides, although only two pixels are shown here, light-emitting layerscorresponding to the respective colors of R (red), G (green) and B(blue) may be individually formed. As an organic EL material used forthe light-emitting layer, a π-conjugated polymer material is used. As atypical polymer material, polyparaphenylene vinylene (PPV),polyvinylcarbazole (PVK), polyfluorene or the like is named.

Although there are various types as the PPV organic EL material, forexample, a material as disclosed in ┌H. Shenk, H. Becker, O. Gelsen, E.Kluge, W. Kreuder, and H. Spreitzer, “Polymers for Light-emittingDiodes”, Euro Display, Proceedings, 1999, p. 33-37┘ or Japanese PatentLaid-Open No. Hei 10-92576 may be used.

Although this embodiment shows the example in which the polymer materialis used as the light-emitting layer, a low molecular organic EL materialmay be used. Besides, as a charge transporting layer or a chargeinjecting layer, an inorganic material such as silicon carbide can alsobe used. As the organic EL material or inorganic material, a well-knownmaterial can be used.

This embodiment adopts the EL layer of a laminate structure in which ahole injecting layer (not shown) made of PEDOT (polythiophene) or PAni(polyaniline) is provided on the light-emitting layer. An anode (notshown) made of a transparent conductive film is provided on the holeinjecting layer. In the case of this embodiment, since light generatedin the light-emitting layer is radiated toward the upper surface side(toward the upper portion of the TFT), the anode must be translucent.Although a compound of indium oxide and tin oxide or a compound ofindium oxide and zinc oxide can be used as the transparent conductivefilm, since the film is formed after the light-emitting layer and thehole injecting layer having low heat resistance are formed, it isdesirable to use a material by which the film can be formed at thelowest possible temperature.

At the point of time when the anode is formed, the EL element iscompleted. Note that the EL element here indicates a capacitor formed ofthe pixel electrode (cathode) 1507, the light-emitting layer, the holeinjecting layer and the anode. As shown in FIG. 15, since the pixelelectrode 1507 is almost coincident with the area of a pixel, the wholepixel functions as the EL element. Thus, a use coefficient of lightemission is very high, and a bright image display becomes possible.

As described above, the EL display panel of the present inventionincludes the pixel portion made of the pixel having the structure asshown in FIG. 15, and includes the switching TFT having an adequatelylow off current value and the current controlling TFT proof against hotcarrier injection. Accordingly, it is possible to obtain the EL displaypanel having high reliability and enabling an excellent image display.

Note that the structure of this embodiment can be freely combined withthe embodiments 1, 2, 6 and 8 and can be carried out.

Embodiment 8

In this embodiment, an example of a film forming apparatus used when anEL layer is formed in the above respective embodiments will bedescribed.

The film forming apparatus of this embodiment will be described withreference to FIG. 19. In FIG. 19, reference numeral 1101 designates aconveying chamber (A), and the conveying chamber (A) 1101 is providedwith a conveying chamber mechanism (A) 1102, in which a substrate 1103is conveyed. The conveying chamber (A) 1101 is made to have a reducedpressure atmosphere, and is shut off from respective processing chambersby gates. Delivery of the substrate to the respective processingchambers is performed by the conveying chamber mechanism (A) when thegate is opened. In order to decrease the pressure in the conveyingchamber (A) 1101, although an exhaust pump such as an oil rotary pump, amechanical booster turbo pump, a turbo molecular pump, or a cryopump canbe used, the cryopump effective in removing moisture is preferable.

In the film forming apparatus of FIG. 19, an exhaust port 1104 isprovided at a side of the conveying chamber (A) 1101, and the exhaustpump is provided under that. When such structure is adopted, there is amerit that the maintenance of the exhaust pump becomes easy.

Hereinafter, the respective processing chambers will be described. Notethat since the conveying chamber (A) 1101 comes to have a reducedatmosphere, all processing chambers directly coupled with the conveyingchamber (A) 1101 are provided with exhaust pumps (not shown). As theexhaust pump, a mechanical booster pump, a turbo molecular pump, or acryopump is used.

First, reference numeral 1105 designates a stock chamber in whichsetting of the substrate is carried out and which is also called a loadlock chamber. The stock chamber 1105 is shut off from the conveyingchamber (A) 1101 by a gate 1100 a, and a carrier (not shown) on whichthe substrate 1103 is set is disposed here. Note that the stock chamber1105 may be divided into a portion for use in carrying a substrate inand a portion for use in carrying a substrate out. Besides, the stockchamber 1105 is provided with the foregoing exhaust pump and a purgeline for introducing a high purity nitrogen gas or rare gas.

In this embodiment, the substrate 1103 is set on the carrier while itselement formation surface is made to face downward. This is forfacilitating a face down system (also called a deposit up system) whenvapor phase film formation (film formation by sputter or evaporation) islater carried out. The face down system is a system in which filmformation is carried out in a state where an element formation surfaceof a substrate faces downward, and according to this system, adhesion ofdust or the like can be suppressed.

Next, reference numeral 1106 designates a conveying chamber (B) which iscoupled with the stock chamber 1105 through a gate 11006 and includes aconveying chamber mechanism (B) 1107. Reference numeral 1108 designatesa baking chamber (bake chamber) which is coupled with the conveyingchamber (B) 1106 through a gate 1100 d. The baking chamber 1108 includesa mechanism for inverting the top and bottom of the surface of thesubstrate. That is, the substrate conveyed in the face down system isonce changed to the face up system here. This is for enabling a nextprocessing in a spin coater 1109 to be carried out in the face upsystem. On the contrary, the substrate which has been subjected to theprocessing in the spin coater 1109 is again returned to the bakingchamber 1108 and is baked, and the top and bottom is again inverted tothe face down system, and the substrate is returned to the stock chamber1105.

The film formation chamber 1109 provided with the spin coater is coupledwith the conveying chamber (B) 1106 through a gate 1100 c. The filmformation chamber 1109 provided with the spin coater is a film formationchamber in which a solution containing an EL material is coated on thesubstrate so that a film containing the EL material is formed, and inthis embodiment, a film of a high molecular (polymer) organic ELmaterial is formed in the film formation chamber 1109 provided with thespin coater. Note that the EL material to be formed into a film includesone used for not only a light-emitting layer but also a charge injectinglayer or a charge transporting layer. Besides, any well-known highmolecular organic EL material may be used.

As a typical organic EL material which becomes the light-emitting layer,a PPV (polyparaphenylene vinylene) derivative, a PVK(polyvinylcarbazole) derivative, or a polyfluorene derivative is named.This is also called a π-conjugated polymer. As the charge injectinglayer, PEDOT (polythiophene) or PAni (polyaniline) is named.

Note that in this embodiment, although the film formation chamber usingthe spin coater is shown, it is not necessary to make limitation to thespin coater, but the film formation chamber may use a dispenser,printing, or ink jet instead of the spin coater. Further, as shown inFIG. 19, a pre-processing chamber 1110, a gas phase film formationchamber 1111, a sealing chamber 1112, an ultraviolet light irradiationmechanism 1113, a delivery chamber 1114, a conveying chamber mechanism(c) 1115 and gates 1100 f, 1100 e and 1100 g can be equipped.

Besides, the film forming apparatus of this embodiment can be used whenthe EL layer is formed in the structure in which any structures of theembodiments 1 to 7 are freely combined.

Embodiment 9

In this embodiment, in a light-emitting device using an organic ELmaterial in which emission brightness of red, blue and green aredifferent, W/L of a current controlling TFT of a pixel for displaying acolor with low emission brightness is made larger than W/L of a currentcontrolling TFT of a pixel for displaying a color with relatively highemission brightness. By the above structure, a drain current of thecurrent controlling TFT of the pixel for displaying the color with thelow emission brightness is made higher than a drain current of thecurrent controlling TFT of the pixel for displaying the color with therelatively high emission brightness.

Thus, in the light-emitting device using the organic EL material inwhich the emission brightness of red, blue and green are different, theamount of current flowing through an EL element for displaying the colorwith the low emission brightness becomes larger than the amount ofcurrent flowing through an EL element for displaying the color with therelatively high emission brightness. By this, it is possible to displayan image having an excellent balance among red, blue and green emissionbrightness.

Note that this embodiment can be used in combination with any otherembodiments.

Embodiment 10

In this embodiment, an example of a structure of a light-emitting deviceof the present invention will be described with reference to FIG. 20.

An insulating film 906 is formed on a substrate 905, and there areformed thereon a p-channel TFT 901 and an n-channel TFT 902 included ina CMOS circuit of a driving circuit (source signal line driving circuitor gate signal line driving circuit), and a switching TFT 903 and acurrent controlling TFT 904 included in a pixel portion.

The p-channel TFT 901 included in the driving circuit includes a sourceregion 907, a drain region 909, and a channel forming region 908.Further, the channel TFT 901 includes the gate insulating film 906 onthe channel forming region 908, and a gate electrode 922 on the gateinsulating film 906. A first interlayer insulating film 927 is providedto cover the gate insulating film 906 and the gate electrode 922.Further, the p-channel TFT 901 includes a source wiring line 928connected to the source region 907 and a drain wiring line 929 connectedto the drain region 909, through contact holes provided in the gateinsulating film 906 and the first interlayer insulating film 927.

The n-channel TFT 902 included in the driving circuit includes a sourceregion 912, a drain region 910, and a channel forming region 911.Further, the re-channel TFT 902 includes the gate insulating film 906 onthe channel forming region 911, and a gate electrode 923 on the gateinsulating film 906. The first interlayer insulating film 927 isprovided to cover the gate insulating film 906 and the gate electrode923. Further, the n-channel TFT 902 includes a source wiring line 930connected to the source region 912 and a drain wiring line 929 connectedto the drain region 910, through contact holes provided in the gateinsulating film 906 and the first interlayer insulating film 927.

The switching TFT 903 included in the pixel portion has a double gatestructure. Note that in this embodiment, although the switching TFT 903has the double gate structure, it may have a single gate structure oranother multigate structure. The switching TFT 903 includes a sourceregion 913, a drain region 917, channel forming regions 914 and 916, andan impurity addition region 915. Further, the switching TFT 903 includesthe gate insulating film 906 on the channel forming regions 914 and 916,and gate electrodes 924 and 925 on the gate insulating film 906. Thefirst interlayer insulating film 927 is provided to cover the gateinsulating film 906 and the gate electrodes 924 and 925. Further, theswitching TFT 903 includes a source wiring line (source signal line) 931connected to the source region 913 and a drain wiring line 932 connectedto the drain region 917, through contact holes provided in the gateinsulating film 906 and the first interlayer insulating film 927.

Reference numeral 957 designates a gate wiring line (gate signal line),which electrically connects the gate electrode 924 of the switching TFT903 to the gate electrode 925. The gate wiring line 957 may be formed ofthe same material as the gate electrodes 924 and 925 of the switchingTFT 903 or may be formed of a different material. By forming the gateelectrodes 924 and 925 from a material easy to precisely work, andforming the gate wiring line 957 from a material having resistance lowerthan the material forming the gate electrodes 924 and 925, it becomespossible to form a light-emitting device having higher definition and alarge screen.

The current controlling TFT 904 included in the pixel portion has asingle gate structure. Note that in this embodiment, although thecurrent controlling TFT 904 has the single gate structure, it may have adouble gate structure or another multigate structure. The currentcontrolling TFT 904 includes a source region 918, a drain region 920,and a channel forming region 919. Further, the current controlling TFT904 includes the gate insulating film 906 on the channel forming regions919, and a gate electrode 926 on the gate insulating film 906. The firstinterlayer insulating film 927 is provided to cover the gate insulatingfilm 906 and the gate electrode 926. Further, the current controllingTFT 904 includes a source wiring line 933 connected to the source region918 and a drain wiring line 934 connected to the drain region 920,through contact holes provided in the gate insulating film 906 and thefirst interlayer insulating film 927.

A second interlayer insulating film 935 is formed to cover the firstinterlayer insulating film 927, the source wiring lines 928, 930, 931,and 933, and the drain wiring lines 929, 932, and 934. A thirdinterlayer insulating film (flattening film) 936 made of an organicresin is formed on the second interlayer insulating film 935.

A pixel electrode 937 connected to the drain wiring line 934 of thecurrent controlling TFT 904 through a contact hole formed in the secondinterlayer insulating film 935 and the third interlayer insulating film936 is formed on the third interlayer insulating film 936. In thisembodiment, it is desirable that the pixel electrode 937 is formed of atransparent electrode, for example, ITO.

Besides, a bank 938 for separating EL layers or light-emitting layersbetween pixels is provided on the source wiring line 931. In thisembodiment, although the bank 938 is provided on the source wiring line931, the present invention is not limited to this. The bank 938 may beprovided on the gate wiring line 957.

An EL layer 939 is provided on the pixel electrode 937. The EL layer canbe formed using a well-known material. A cathode 940 is provided on theEL layer 939. The cathode 940 can be formed of a well-known material,and it was formed using MgAg in this embodiment.

It is desirable that the EL layer 939 and the cathodes 940 arecontinuously formed in the same chamber without opening to the air.

Since the light-emitting device having the structure of this embodimentdoes not includes an LDD region, the operation speed is relatively high.

In the case where a voltage applied to the EL element becomes 10 V orless, preferably 5 V or less, since deterioration of the TFT due to thehot carrier effect does not become a serious problem, the structureincluding no LDD regions set forth in this embodiment is effective insuppressing the number of fabricating steps.

Embodiment 11

In this embodiment, a description will be given of an example in thecase where the present invention is applied to an actual light-emittingdevice using the above expressions 8 and 11 and which is different fromthe former embodiment.

In this embodiment, a light-emitting device having a resolution of QVGAof 320×240 and a size of 4 inches will be exemplified.

A pixel size of the 4-inch QVGA light-emitting device is about 84 μm×252μm. When an attempt to obtain constant brightness is made, the amount ofcurrent per unit area flowing through an EL element is determined. Inthis embodiment, it is made 3 mA/cm² per unit area.

Thus, a drain current Id of a current controlling TFT included in eachof pixels is expressed by the following expression 29.

[Expression 29]

The above expression 29 indicates a value of the drain current Id of thecurrent controlling TFT when the opening ratio of the light-emittingdevice is made 100%. Actually, in almost all cases, the opening ratio ofthe light-emitting device is not 100%. As the opening ratio of thelight-emitting device becomes small, the value of the actually requireddrain current Id becomes large. For example, if the opening ratio of thelight-emitting device of this embodiment is 30%, the value of theactually required drain current Id is obtained by the following equation30.

[Expression 30]

Since the light-emitting device used in this embodiment uses a bottomgate type current controlling TFT, when a mobility of the currentcontrolling TFT is μ=50 (m²/V·sec) and a capacitance value of the gatecapacitance is Co=2.4×10⁻⁸ (F/cm²), a constant A is obtained fromexpression 31.

[Expression 31]

In this embodiment, a difference between emission brightness of therespective pixels is restricted within a range of, for example, ±5%.When a gate voltage Vgs_((max)) immediately before the TFT is broken ismade 25 V, and a value of a threshold voltage Vth is made 0 V, thefollowing expressions 32 and 33 are obtained from the expressions 8 and11.

[Expression 32]

[Expression 33]

In the light-emitting device of the present invention, the values ofΔVth and W/L are determined within the range where the above expression32 or 33 is satisfied, and the fluctuation of the drain current Id canbe suppressed to the range of ±5%.

It is generally desirable that the fluctuation ΔVth of the thresholdvalue of the current controlling TFT is 0.1 V or less.

It is assumed that the fluctuation ΔVth of the threshold voltage isΔVth≦0.1 V by a fabricating process of the TFT. When ΔVth=0.1 V issubstituted in the expression 33, the following expression 34 isobtained.

[Expression 34]

If the ratio W/L of the channel length L to the channel width W isdetermined so that the expression 34 is satisfied, the fluctuation ofthe drain current Id can be suppressed to the range of ±5%.

According to the above structure, in the light-emitting device of thepresent invention, the number of thin film transistors provided in eachof pixels is made two to prevent a drop in the opening ratio, and itbecomes possible to suppress uneven brightness due to fluctuation in thethreshold voltage of current controlling TFTs included in the respectivepixels.

Note that in this embodiment, although the description has been given ofthe example in which the fluctuation of the drain current Id issuppressed to the range of ±5%, the present invention is not limited tothis numerical value.

Embodiment 12

In the present invention, external luminous quantum efficiency can beremarkably improved by using an EL material which can usephosphorescence from a triplet exciton for light emission. By this, itbecomes possible to realize low power consumption, long lifetime, andlight weight of an EL element.

Here, there is a report in which the triplet exciton is used and theexternal luminous quantum efficiency is improved. (T. Tsutsui, C.Adachi, S. Saito, Photochemical Processes in Organized MolecularSystems, ed. K. Honda, (Elsevier Sci. Pub., Tokyo, 1991) p. 437.)

A molecular formula of an EL material (coumarin pigment) reported in theabove paper is as follows:

[Chemical Formula 1]

-   (M. A. Baldo, D. F. O'Brien, Y. You, A. Shoustikov, S. Sibley, M. E.    Thompson, S. R. Forrest, Nature 395 (1988) p. 151.)

A molecular formula of an EL material (Pt complex) reported in the abovepaper is as follows:

[Chemical Formula 2]

-   (M. A. Baldo, S. Lamansky, P. E. Burrrows, M. E. Thompson, S. R.    Forrest, Appl. Phys. Lett., 75 (1999) p. 4.) (T. Tsutsui, M.-J.    Yang, M. Yahiro, K. Nakamura, T. Watanabe, T. Tusji, Y. Fukuda, T.    Wakimoto, S. Mayaguchi, Jpn. Appl. Phys., 38 (12B) (1999) L1502.)

A molecular formula of an EL material (Ir complex) reported in the abovepaper is as follows:

[Chemical Formula 3]

As described above, if phosphorescence emission from the triplet excitoncan be used, in principle, it becomes possible to realize the externalluminous quantum efficiency 3 to 4 times as high as the case of usingfluorescence from a single exciton.

Note that the structure of this embodiment can be freely combined withany structure of the embodiments 1 to 11 and can be carried out.

Embodiment 13

A light-emitting device has superior visibility in bright locations incomparison to a liquid crystal display device because it is of aself-emitting type, and moreover viewing angle is wide. Accordingly, itcan be used as a display portion for various electronic apparatuses. Forexample, it is appropriate to use the light-emitting device of thepresent invention as a display portion of an EL display (a displayincorporating the light-emitting device in its casing) having a diagonalequal to 30 inches or greater (typically equal to 40 inches or greater)for appreciation of TV broadcasts by a large screen.

Note that all displays exhibiting (displaying) information such as apersonal computer display, a TV broadcast reception display, or anadvertisement display are included as the EL display. Further, thelight-emitting device of the present invention can be used as a displayportion of the other various electronic apparatuses.

The following can be given as examples of such electronic apparatuses: avideo camera; a digital camera; a goggle type display (head mounteddisplay); a car navigation system; an audio reproducing device (such asa car audio system, an audio compo system); a notebook personalcomputer; a game equipment; a portable information terminal (such as amobile computer, a mobile telephone, a mobile game equipment or anelectronic book); and an image reproduction device provided with arecording medium (specifically, a device which performs reproduction ofa recording medium and is provided with a display which can displaythose images, such as a digital video disk (DVD)). In particular,because portable information terminals are often viewed from a diagonaldirection, the wideness of the field of vision is regarded as veryimportant. Thus, it is preferable that the light-emitting device isemployed. Examples of these electronic instruments are shown in FIGS.16A through 17B.

FIG. 16A illustrates an EL display which includes a frame 2001, asupport table 2002, a display portion 2003, or the like. Thelight-emitting device in accordance with the present invention can beused as the display portion 2003. The light-emitting device is of aself-emitting type and therefore requires no back light. Thus, thedisplay portion thereof can have a thickness thinner than that of theliquid crystal display device.

FIG. 16B illustrates a video camera which includes a main body 2101, adisplay portion 2102, an audio input portion 2103, operation switches2104, a battery 2105, an image receiving portion 2106, or the like. Thelight-emitting device in accordance with the present invention can beused as the display portion 2102.

FIG. 16C illustrates a portion (the right-half piece) of anelectro-optical device of head-mounted type which includes a main body2201, signal cables 2202, a head mount band 2203, a screen portion 2204,an optical system 2205, a display portion 2206, or the like. Thelight-emitting device in accordance with the present invention can beused as the display portion 2206.

FIG. 16D illustrates an image reproduction apparatus which includes arecording medium (more specifically, a DVD reproduction apparatus),which includes a main body 2301, a recording medium (a DVD or the like)2302, operation switches 2303, a display portion (a) 2304, anotherdisplay portion (h) 2305, or the like. The display portion (a) 2304 isused mainly for displaying image information, while the display portion(b) 2305 is used mainly for displaying character information. Thelight-emitting device in accordance with the present invention can beused as these display portions (a) and (b), 2304 and 2305. The imagereproduction apparatus including a recording medium further includes adomestic game equipment or the like.

FIG. 16E illustrates a goggle type display (head-mounted display) whichincludes a main body 2401, a display portion 2402, an arm portion 2403.The light-emitting device in accordance with the present invention canbe used as the display portion 2402.

FIG. 16F illustrates a personal computer which includes a main body2501, a frame 2502, a display portion 2503, a key board 2504, or thelike. The light-emitting device in accordance with the present inventioncan be used as the display portion 2503.

Note that if emission brightness of an EL material becomes higher in thefuture, it will be applicable to a front-type or rear-type projector inwhich light including output image information is enlarged by means oflenses or the like to be projected.

The above mentioned electronic apparatuses are more likely to be usedfor display information distributed through a telecommunication pathsuch as Internet, a CATV (cable television system), and in particularlikely to display moving picture information. The light-emitting deviceis suitable for displaying moving pictures since the EL material canexhibit high response speed.

Further, since a light emitting portion of the light-emitting deviceconsumes power, it is desirable to display information in such a mannerthat the light emitting portion therein becomes as small as possible.Accordingly, when the light-emitting device is applied to a displayportion which mainly displays character information, e.g., a displayportion of a portable information terminal, and more particular, aportable telephone or an audio reproducing device, it is desirable todrive the light emitting device so that the character information isformed by a light-emitting portion while a non-emission portioncorresponds to the background.

FIG. 17A illustrates a portable telephone which includes a main body2601, an audio output portion 2602, an audio input portion 2603, adisplay portion 2604, operation switches 2605, and an antenna 2606. Thelight-emitting device in accordance with the present invention can beused as the display portion 2604. Note that the display portion 2604 canreduce power consumption of the portable telephone by displayingwhite-colored characters on a black-colored background.

Further, FIG. 17B illustrates a sound reproduction device, specifically,a car audio equipment in concrete term, which includes a main body 2701,a display portion 2702, and operation switches 2703 and 2704. Thelight-emitting device in accordance with the present invention can beused as the display portion 2702. Although the car audio equipment ofthe mount type is shown in the present embodiment, the present inventionis also applicable to a portable type or domestic sound reproducingdevice. The display portion 2702 can reduce power consumption bydisplaying white-colored characters on a black-colored background, whichis particularly advantageous for the portable type sound reproductiondevice.

As set forth above, the present invention can be applied variously to awide range of electronic instruments in all fields. The electronicapparatuses in the present embodiment can be obtained by utilizing alight-emitting device having the configuration in which the structuresin Embodiments 1 through 12 are freely combined.

According to the present invention, in the case where the fluctuationΔVth of the threshold voltage is fixed by a fabricating process of aTFT, from the value of the fluctuation ΔVth of the threshold voltage,the range of the ratio W/L of the channel width W to the channel lengthL is determined by the expression 14.

Besides, according to the present invention, in the case where the valueof the ratio W/L of the channel width W to the channel length L is fixedby a problem of design, from the value of the ratio W/L of the channelwidth W to the channel length L, the range of the fluctuation ΔVth ofthe threshold voltage is determined by the expression 15.

According to the above structure, in the light-emitting device of thepresent invention, the number of thin film transistors provided in eachof the pixels is made two to prevent a drop in the opening ratio, and itbecomes possible to suppress uneven luminance due to fluctuation in thethreshold voltage of current controlling TFTs included in the respectivepixels.

1. (canceled)
 2. A semiconductor device comprising: a first transistorof a first pixel comprising a first channel forming region, the firstchannel forming region having a first channel width and a first channellength; and a second transistor of a second pixel comprising a secondchannel forming region, the second channel forming region having asecond channel width and a second channel length; and a flattening layerover the first channel forming region and the second channel formingregion; a first pixel electrode of the first pixel electricallyconnected to the first transistor, the first pixel electrode being overthe flattening layer; a second pixel electrode of the second pixelelectrically connected to the second transistor the second pixelelectrode being over the flattening layer; a first layer including anorganic light emitting material, the first layer being over the firstpixel electrode; and a second layer including an organic light emittingmaterial, the second layer being over the second pixel electrode,wherein a ratio of the first channel width to the first channel lengthis different from a ratio of the second channel width to the secondchannel length.
 3. The semiconductor device according to claim 2,wherein the first layer is configured to emit light of a first color,the second layer is configured to emit light of a second color, and thefirst color is different from the second color.
 4. The semiconductordevice according to claim 2, wherein each of the first transistor andthe second transistor is a thin film transistor.
 5. The semiconductordevice according to claim 2, wherein each of the first transistor andthe second transistor is a current controlling thin film transistor. 6.The semiconductor device according to claim 2, wherein each of the firsttransistor and the second transistor is a top-gate transistor.
 7. Thesemiconductor device according to claim 2, wherein each of the firsttransistor and the second transistor includes a poly-crystallinesemiconductor film.
 8. The semiconductor device according to claim 2,wherein the first pixel further comprises a first switching transistor,and wherein the second pixel further comprises a second switchingtransistor.
 9. The semiconductor device according to claim 2, whereinthe ratio of the first channel width to the first channel length isequal to or larger than 2.26×10⁻³ and equal to or smaller than 0.214.10. An electronic apparatus having the semiconductor device according toclaim 2, wherein the electronic apparatus is one selected from the groupconsisting of a video camera, a digital camera, a goggle type display, acar navigation system, an audio reproduction device, a notebook personalcomputer, a game equipment, a mobile computer, a mobile telephone, amobile game equipment, an electronic book, and an image reproductiondevice provided with a recording medium.
 11. A semiconductor devicecomprising: a first transistor of a first pixel comprising a firstchannel forming region, the first channel forming region having a firstchannel width and a first channel length; and a second transistor of asecond pixel comprising a second channel forming region, the secondchannel forming region having a second channel width and a secondchannel length; and a first signal line configured to supply a signal tothe first transistor; a second signal line configured to supply a signalto the second transistor, the second signal line extending in parallelwith the first signal line; a current supply line extending between andin parallel with the first signal line and the second signal line, thecurrent supply line being electrically connected to the first transistorand the second transistor, a flattening layer over the first channelforming region and the second channel forming region; a first pixelelectrode of the first pixel electrically connected to the firsttransistor, the first pixel electrode being over the flattening layer; asecond pixel electrode of the second pixel electrically connected to thesecond transistor the second pixel electrode being over the flatteninglayer; a first layer including an organic light emitting material, thefirst layer being over the first pixel electrode; and a second layerincluding an organic light emitting material, the second layer beingover the second pixel electrode, wherein a ratio of the first channelwidth to the first channel length is different from a ratio of thesecond channel width to the second channel length.
 12. The semiconductordevice according to claim 11, wherein the first layer is configured toemit light of a first color, the second layer is configured to emitlight of a second color, and the first color is different from thesecond color.
 13. The semiconductor device according to claim 11,wherein each of the first transistor and the second transistor is a thinfilm transistor.
 14. The semiconductor device according to claim 11,wherein each of the first transistor and the second transistor is acurrent controlling thin film transistor.
 15. The semiconductor deviceaccording to claim 11, wherein each of the first transistor and thesecond transistor is a top-gate transistor.
 16. The semiconductor deviceaccording to claim 11, wherein each of the first transistor and thesecond transistor includes a poly-crystalline semiconductor film. 17.The semiconductor device according to claim 11, wherein the first pixelfurther comprises a first switching transistor, and wherein the secondpixel further comprises a second switching transistor.
 18. Thesemiconductor device according to claim 11, wherein the ratio of thefirst channel width to the first channel length is equal to or largerthan 2.26×10⁻³ and equal to or smaller than 0.214.
 19. A semiconductordevice comprising: a first transistor of a first pixel comprising afirst channel forming region, the first channel forming region having afirst channel width and a first channel length; and a second transistorof a second pixel comprising a second channel forming region, the secondchannel forming region having a second channel width and a secondchannel length; and a gate signal line configured to supply a signal tothe first pixel and the second pixel; a current supply line extending inparallel with the gate signal line, the current supply line beingelectrically connected to the first transistor and the secondtransistor, a flattening layer over the first channel forming region andthe second channel forming region; a first pixel electrode of the firstpixel electrically connected to the first transistor, the first pixelelectrode being over the flattening layer; a second pixel electrode ofthe second pixel electrically connected to the second transistor thesecond pixel electrode being over the flattening layer; a first layerincluding an organic light emitting material, the first layer being overthe first pixel electrode; and a second layer including an organic lightemitting material, the second layer being over the second pixelelectrode, wherein a ratio of the first channel width to the firstchannel length is different from a ratio of the second channel width tothe second channel length.
 20. The semiconductor device according toclaim 19, wherein the first layer is configured to emit light of a firstcolor, the second layer is configured to emit light of a second color,and the first color is different from the second color.
 21. Thesemiconductor device according to claim 19, wherein each of the firsttransistor and the second transistor is a thin film transistor.
 22. Thesemiconductor device according to claim 19, wherein each of the firsttransistor and the second transistor is a current controlling thin filmtransistor.
 23. The semiconductor device according to claim 19, whereineach of the first transistor and the second transistor is a top-gatetransistor.
 24. The semiconductor device according to claim 19, whereineach of the first transistor and the second transistor includes apoly-crystalline semiconductor film.
 25. The semiconductor deviceaccording to claim 19, wherein the first pixel further comprises a firstswitching transistor, and wherein the second pixel further comprises asecond switching transistor.
 26. The semiconductor device according toclaim 19, wherein the ratio of the first channel width to the firstchannel length is equal to or larger than 2.26×10⁻³ and equal to orsmaller than 0.214.